Metrology Method for a Semiconductor Manufacturing Process

ABSTRACT

A method and apparatus is disclosed for monitoring critical dimensions in a pattern of 1-dimensional and/or 2-dimensional features, produced on a substrate in a process step that is part of or related to a manufacturing process for producing a semiconductor device, the process step being performed in accordance with a predefined pattern design, wherein one or more metrology targets (1) are added to the pattern design. The targets comprise one or more versions of an asymmetric metrology mark, each version of the mark comprising a uniform portion (2) and a periodic portion (3), the latter comprising a regular array of parallel line-shaped features or an array of 2-dimensional features. The design width of the features is situated in a range situated around a nominal design width w0. A position-related parameter S is defined that is essentially proportional to the design widths in the range. Determination of the shift δS of the S parameter with respect to a pre-defined process operating point, allows to assess the critical dimension of the features produced by the process step.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a non-provisional patent application claimingpriority to European Provisional Patent Application No. EP 17191242.1,filed Sep. 15, 2017; European Provisional Patent Application No. EP17164341.4, filed Mar. 31, 2017; and European Provisional PatentApplication No. EP 17164371.1, filed Mar. 31, 2017, the contents of eachof which are hereby incorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure is related to semiconductor processing, inparticular to a metrology method for monitoring lithographic and/or etchprocesses during the fabrication of a semiconductor chip.

BACKGROUND

Semiconductor processing includes many patterning steps, wherein adevice design is first transferred to a lithographic mask and the maskis exposed to a light source, resulting in the printing of the patternon a photoresist film deposited on a layer stack that is built up layerby layer on a semiconductor wafer. After development of the resist,etching transfers the pattern to a layer of the stack, for example forrealizing a metal conductor pattern in a level of the back-end-of-linestack of an integrated circuit chip. Subsequent processing may alsoinclude additional deposition, etch and polish steps to further modifythe pattern, such as in self-aligned multiple patterning. Featuredimensions of the patterns in present day processing technology are ofthe order of nanometers, and the monitoring of the features includesspecific metrology tools. The main dimension of interest is referred toas the critical dimension (CD) of rectilinear features of physicalpatterns at each of the various processing steps.

Current methods for measuring CD variation have significant drawbacks:(a) High resolution methods such as Scanning Electron Microscopy (SEM)or Atomic Force Microscopy (AFM) are relatively slow, expensive, andpotentially destructive (particularly to resist patterns). Currentoptical methods (e.g., scatterometry) include model development for eachpattern and process configuration, are difficult to calibrate andsusceptible to variation in underlying film stacks. There may be a needtherefore for alternative methods that allow a reliable andnon-intrusive way of verifying the critical dimensions of printed and/oretched pattern features.

Self-aligned multi-patterning SAxP is a method essential tosemiconductor density scaling. It is a technique of multiplying,typically doubling (SADP) or quadrupling (SAQP), the spatial periodicityof regular arrays by the formation of sidewall spacers on a periodicmandrel (aka, core) structure. To achieve uniform periodicity in thefinal array, the width dimension of the mandrel elements might becontrolled relative to their pitch and to the spacer width. In the SAQPexample, the mandrel pitch may be 128 nm, the width of the spacer 16nm,so that the mandrel element width might be controlled at 48 nm to resultin an SAQP pitch of 32 nm. A variation of the mandrel width results in avariation of the nominal 32 nm pitch across the array, known as “pitchwalking.” Pitch walking is a significant failure mechanism insemiconductor manufacturing. Current methods of pitch walking metrologyand control are slow, expensive and error-prone.

SUMMARY

The disclosure is related to a method as disclosed in the appendedclaims. A method and apparatus is disclosed for monitoring criticaldimensions in patterns produced on a substrate in a process step that ispart of or related to a manufacturing process for producing asemiconductor device, such as an integrated circuit chip, the processstep being performed in accordance with a predefined pattern design,wherein one or more metrology targets are added to the pattern design.The targets may for example be embedded in or on the periphery of theactive device area of a lithographic mask used in a chip manufacturingprocess, or the targets may be added to test patterns produced on a testwafer in a tool or process qualification and calibration sequence. Theterm “pattern design” is therefore not to be understood as referringonly to device designs applied in chip manufacturing. Some embodimentsfind an important application in such manufacturing processes but alsoin related processes, such as the above-describedqualification/calibration, and the production of lithographic masks.

Each target included in the pattern design comprises one or moreversions of an asymmetric metrology mark, each version of the markcomprising a uniform portion and a periodic portion, the lattercomprising a regular array of parallel 1-dimensional features, i.e.line-shaped features or a regular array of 2-dimensional features, i.e.an array of squares or rectangles. According to an embodiment, thedesign dimensions of the features, for example the design width w ofline features are distributed across a range situated around andincluding a nominal value w₀ that corresponds to a critical dimension ofthe features produced in the manufacturing process. From the detectedposition of the centroids of the mark patterns, a position-relatedparameter S is derived that is essentially proportional to the designdimensions in the range. According to example embodiments, the shift δSof this parameter is determined with respect to a process operatingpoint previously determined during setup and calibration of theproduction process. According to embodiments of the method, theproportionality factor of the relation between S and the designdimension may also be determined, as well as the shift δCD of thecritical dimensions of features printed in the fabrication process, withrespect to a reference value CD_(p) measured at the process operatingpoint. The asymmetric mark design enables proportionality factors>>1;i.e., amplification of response of S to δCD such that 6S>>δCD and theeffective precision of CD determination is much better than theintrinsic precision of the measurement itself.

The disclosure is more particularly related to a method for monitoringthe critical dimensions of rectilinear features, i.e. 1-dimensionalfeatures, and/or 2-dimensional features, arranged in a pattern andproduced on a substrate by a process step that is part of or related toa manufacturing process for producing a semiconductor device, theprocess step being performed in accordance with a predefined patterndesign, wherein one or more metrology targets are added to the patterndesign, each target comprising one or more marks of an asymmetricdesign, each mark comprising a uniform portion of width K and a periodicportion of width L arranged adjacently in a given direction, theperiodic portion comprising 1-dimensional or 2-dimensional featuresarranged in a regular array, wherein the periodic portion comprises anarray of 1-dimensional features at a design pitch (p) and designdimension equal to and/or situated in a range around a nominal value(w₀) or 2-dimensional features at design pitches (p_(a), p_(b)) anddesign dimensions equal to and/or situated in a range around nominalvalues (w₀ _(a) , w₀ _(b) ). In other words, the periodic portion mayinclude an array of 1-dimensional features or 2-dimensional features ata design pitch (p; p_(a), p_(b)), wherein the design dimension of the1-dimensional features or one of the design dimensions of the2-dimensional features is equal to and/or situated in a range around anominal value (w₀). The nominal value w₀ may thus be the nominal valueof the width of the 1-dimensional features, or it may be the nominalvalue of the width in one or the other dimension, of the 2-dimensionalfeatures.

The method comprises the following steps, performed in relation to eachof the one or more targets:

-   -   Performing the process step, thereby obtaining one or more        asymmetric mark patterns corresponding respectively to the one        or more asymmetric marks. The term “corresponding to” means that        the mark patterns are reproductions of the marks.    -   Defining a parameter S representative of the position of the        centroid of the mark patterns, wherein the parameter S changes        essentially linearly as a function of at least one of the design        dimensions w of the of the 1-dimensional or 2-dimensional        features, i.e. dS/dw=α with α a proportionality factor,    -   determining from the mark patterns the shift δS of the        S-parameter for an asymmetric mark having features of design        dimension w₀ with respect to a reference value for the        S-parameter, the reference value being valid at a previously        defined process operating point for the process step,    -   Assessing on the basis of δS the critical dimension of        1-dimensional and/or 2-dimensional features of the one or more        mark patterns produced by the process step.

According to an embodiment, the method comprises for each target:

-   -   determining the proportionality factor α,    -   Calculating δw₀=δS/α,    -   Evaluating δw₀ with respect to a tolerance.

The method may further comprise the step of evaluating |α−α₀| withrespect to a tolerance, wherein α₀ is the value for α at the processoperating point.

According to an embodiment:

-   -   one or more of the targets comprises asymmetric marks configured        so that the position of the centroid of the asymmetric mark        patterns is detectable by an optical tool, so that the parameter        S can be measured with the help of the tool,    -   the one or more targets comprise a plurality of the asymmetric        marks, having the one or more design dimensions in a range        around the nominal value w₀, and possibly including a mark        having the design dimensions equal to the nominal value w₀,    -   for each of the one or more targets:        -   the parameter α is determined by fitting a linear function            to the measured values of the S-parameter for the plurality            of marks,        -   δS is determined as S₀−S_(0ref), wherein:            -   S₀ is the parameter value for a mark pattern of a mark                having features of the nominal design dimension,            -   S_(0ref) is the reference value for the S-parameter at                the process operating point.

According to an embodiment, the one or more asymmetric marks arearranged in each of the one or more targets as mirrored pairs ofidentical marks, mirrored about a line perpendicular to the direction inwhich the uniform and periodic portion are arranged, and wherein theposition-representative parameter S is a function of the distance Dbetween the centroids of the two mirrored mark patterns representativeof the two mirrored marks.

According to an embodiment, the one or more asymmetric marks arearranged in each of the one or more targets with respect to one or moresymmetric marks, and wherein the position-representative parameter S isa function of the distance D between the centroid of the asymmetric markpatterns representative of the one or more asymmetric marks and thecentroid of the symmetric mark pattern or patterns representative of orderived from the one or more symmetric marks.

According to an embodiment, the position representative parameter S isequal to D−D_(s), with D_(s) the design value of the distance betweenthe geometric midpoints of two mirrored marks, wherein D_(s) is aconstant for each of the plurality of design widths (w₀, w⁻¹, w₊₁, etc.)and for the given direction.

According to an embodiment:

-   -   one or more of the targets comprises at least a first and a        second set of diffraction gratings, each set being formed of two        grating marks arranged adjacently in the same direction and in a        repeated manner, at least one of the grating marks in each set        being an asymmetric mark as defined in the preceding claims,    -   the asymmetric mark of the first grating set comprising features        of design dimension w₀−Δw₀, wherein Δw₀ is a predefined offset        value,    -   the asymmetric mark of the second grating set comprises features        of design dimension w₀+Δw₀,    -   δS and α are calculated from the equations:

α=(ΔI ₁ −ΔI ₂)/2κΔw ₀

δS=(ΔI ₁ +ΔI ₂)/2κ

-   -   wherein:        -   ΔI₁ and ΔI₂ are the differences between the plus and minus            first order diffracted intensity measured on the first and            second set of mark features obtained by the lithography step            or the etch step, on the basis of the first and second            grating set,        -   κ is a diffraction factor.

According to an embodiment, κ is calculated from a measurement of theplus and minus diffraction intensities of an additional set of twogratings located within or in the vicinity of the pattern design, eachgrating set formed of two marks arranged adjacently and in a repeatedmanner, at least one of the marks being an asymmetric mark, and whereinthe distance between two adjacent marks A′ and B′ is different in thefirst grating compared to the second grating, the difference between thedistances being pre-defined.

The method may further comprise for each target and for at least one ofthe dimensions:

-   -   Calculating from δS the shift δCD of the critical dimension CD        of features of the nominal design dimension w₀, with respect to        the value of the critical dimension CD_(p) at the process        operating point, and wherein δCD is calculated as β.δw₀, wherein        β is a proportionality factor that expresses a linear relation        between CD and the design dimension of the features, i.e.        dCD/dw=β,    -   evaluating δCD with respect to a tolerance.

According to an embodiment, the factor β is calculated as (α/G) whereinG is proportional to L/p, with L the width in the direction of asymmetryof the periodic portion of the mark pattern or patterns and p thenominal pitch of the line features of the periodic portion.

The method may further comprise the step of evaluating |β−1| withrespect to a tolerance.

According to an embodiment, one or more of the targets comprises one ormore asymmetric marks configured so that the position of the centroid ofthe asymmetric mark patterns is detectable by an optical tool, so thatthe parameter S can be measured with the help of the tool, and whereinat least one target comprises a mark having at least one of the designdimensions equal to the nominal value, and wherein the method furthercomprises the steps of:

-   -   Calculating from δS the shift δCD of the critical dimension CD        of features of the nominal design dimension w₀, with respect to        the value of the critical dimension CD_(p) at the process        operating point, and wherein δCD is calculated as δS/G, wherein        G is proportional to L/p with L the width of the periodic        portion of the mark and p the pitch of the array of features in        the direction of the dimension,    -   evaluating δCD with respect to a tolerance.

According to an embodiment, the one or more asymmetric marks in at leastone of the targets are comb-shaped marks, wherein the periodic portioncomprises an array of parallel features extending in the direction inwhich the uniform and periodic portions are arranged.

According to an embodiment, the one or more asymmetric marks in at leastone of the targets are rail-shaped marks, wherein the periodic portioncomprises an array of parallel features extending in the directionperpendicular to the direction in which the uniform and the periodicportion are arranged.

According to an embodiment, at least one of the targets comprises afirst group of asymmetric marks of which the uniform and periodicportions are arranged in a first direction, and a second group of thesame asymmetric marks, of which the uniform and periodic portion arearranged in a second direction perpendicular to the first, and whereinδS is determined for each of the two directions.

According to an embodiment, one or more of the targets comprisesasymmetric marks configured so that the position of the centroid of theasymmetric mark patterns is detectable by an optical tool, so that theparameter S can be measured with the help of the tool, and wherein themarks are arranged in cross-shapes, each cross comprising four identicalmarks.

The target may comprise at least one first area and at least one secondarea, the marks in the second area being mirrored with respect to themarks in the first area.

According to an embodiment, the process step is a lithography step usinga lithographic mask or an etch step following the lithography step andwherein the one or more targets are included in the lithographic mask.

According to an embodiment, the process step is a step of producing thedevice design on the lithographic mask.

The disclosure is furthermore related to a method for monitoringcritical dimensions of 1-dimensional and/or 2-dimensional featuresarranged in a pattern and produced on a substrate by an initial processstep that is part of or related to a manufacturing process for producinga semiconductor device, the process step being performed in accordancewith a predefined pattern design, wherein one or more metrology targetsare added to the pattern design, each target comprising one or moremarks (1) of an asymmetric design, each mark comprising a uniformportion of width K (2) and a periodic portion of width L (3) arrangedadjacently in a given direction (x,y), the periodic portion (3)comprising 1-dimensional or 2-dimensional features arranged in a regulararray, wherein the periodic portion comprises an array of 1-dimensionalfeatures or 2-dimensional features at a design pitch (p; p_(a), p_(b))and wherein the design dimension of the 1-dimensional features or one ofthe design dimensions of the 2-dimensional features is equal to and/orsituated in a range around a nominal value (w₀), and wherein the methodfurther comprises additional process steps after the initial processstep, thereby obtaining one or more asymmetric mark patterns notdirectly corresponding to the one or more asymmetric marks but derivedfrom the one or more asymmetric marks,

wherein the method comprises the following steps, performed in relationto each of the one or more targets:

-   -   Performing the initial process step, thereby obtaining one or        more initial asymmetric mark patterns corresponding respectively        to the one or more asymmetric marks,    -   Defining a parameter S representative of the position of the        centroid of the initial mark patterns, wherein the parameter S        changes essentially linearly as a function of at least one of        the design dimensions w of the of the 1-dimensional or        2-dimensional features, i.e. dS/dw=α with α a proportionality        factor,    -   determining from the initial mark patterns the shift δS of the        S-parameter for an asymmetric mark having features of design        dimension w₀ with respect to a reference value for the        S-parameter, the reference value being valid at a previously        defined process operating point for the process step,    -   Assessing on the basis of δS the critical dimension of        1-dimensional and/or 2-dimensional features of the one or more        initial mark patterns produced by the process step,        and/or wherein the method comprises the following steps,        performed in relation to each of the one or more targets:    -   Defining a parameter S representative of the position of the        centroid of the derived mark patterns, wherein the parameter S        changes essentially linearly as a function of at least one of        the design dimensions w of the of the 1-dimensional or        2-dimensional features, i.e. dS/dw=α with α a proportionality        factor,    -   For one or more of the additional process steps:        -   determining from the derived mark patterns the shift δS of            the S-parameter for an asymmetric mark having features of            design dimension w₀ with respect to a reference value for            the S-parameter, the reference value being valid at a            previously defined process operating point for the            additional process step,        -   Assessing on the basis of δS the critical dimension of            1-dimensional and/or 2-dimensional features of the one or            more initial mark patterns.

According to an embodiment of the method described in the previousparagraph, the method is applied for verifying pitch walk errors duringthe manufacturing process of arrays of semiconductor device features byself-aligned multiple patterning, where the manufacturing processincludes the steps of producing—by lithography and etching—a mandrelstructure that serves as a starting point for the self-aligned multiplepatterning, wherein the lithography steps include the exposure of aresist layer through a lithographic mask, wherein the initial processstep is the lithography step or the etch step and wherein the one ormore additional process steps are the one or more steps of producingself-aligned mark patterns derived from the mandrel structure, andwherein the lithographic mask comprises one or more metrology targets,each target comprising one or more comb-shaped metrology marks, designedto produce mandrel structures comprising a base portion and a periodicportion, the periodic portion comprising a mandrel array of parallelfeatures of equal design width w₀ extending outward from the baseportion, the arrays of the mandrel structures having the same pitch p,and one or more different design widths of the parallel features, andwherein the derived mark patterns obtained by self-aligned multiplepatterning on the basis of the mandrel structures are serpentine-shapedstructures comprising arrays of increased periodicity at eachself-aligned patterning step, with the position of the centroid of theserpentine-shaped structures in a direction transverse to theperiodicity of the arrays being sensitive to the width of the parallelfeatures, and wherein the method comprises determining the positions ofthe centroids of the serpentine structures and/or of the centroids ofthe mandrel structures, and evaluating the positions with respect to acondition of no or essentially no pitch walk errors occurring in one ormore of the arrays of semiconductor device features produced in themanufacturing process.

According to an embodiment of the method including one or moreadditional process steps, the method comprises for each target:

-   -   determining the proportionality factor α,    -   Calculating δw₀=δS/α,    -   Evaluating δw₀ with respect to a tolerance.

The method including one or more additional process steps may furthercomprise the step of evaluating |α−α₀| with respect to a tolerance,wherein α₀ is the value for α at the process operating point for theinitial process or for the additional process step.

According to an embodiment of the method including one or moreadditional process steps:

-   -   one or more of the targets comprises asymmetric marks configured        so that the position of the centroid of the asymmetric mark        patterns is detectable by an optical tool, so that the parameter        S can be measured with the help of the tool,    -   where the one or more targets include a plurality of the        asymmetric marks, having the one or more design dimensions in a        range around the nominal value w₀, and possibly including a mark        having the design dimensions equal to the nominal value w₀,    -   for each of the one or more targets:        -   the parameter α is determined by fitting a linear function            to the measured values of the S-parameter for the plurality            of marks,        -   δS is determined as S₀−S_(0ref), wherein:            -   S₀ is the parameter value for a mark pattern of a mark                having features of the nominal design dimension,            -   S_(0ref) is the reference value for the S-parameter at                the process operating point for the initial process step                or for the additional process step.

According to an embodiment of the method including one or moreadditional process steps, the one or more asymmetric marks are arrangedin each of the one or more targets as mirrored pairs of identical marks,mirrored about a line perpendicular to the direction in which theuniform and periodic portion are arranged, and wherein theposition-representative parameter S is a function of the distance Dbetween the centroids of the two mirrored mark patterns representativeof or derived from the two mirrored marks.

According to an embodiment of the method including one or moreadditional process steps, the one or more asymmetric marks are arrangedin each of the one or more targets with respect to one or more symmetricmarks, and wherein the position-representative parameter S is a functionof the distance D between the centroid of the asymmetric mark patternsrepresentative of or derived from the one or more asymmetric marks andthe centroid of the symmetric mark pattern or patterns representative ofor derived from the one or more symmetric marks.

According to an embodiment of the method including one or moreadditional process steps, the position representative parameter S isequal to D−D_(s), with D_(s) the design value of the distance betweenthe geometric midpoints of two mirrored marks, wherein D_(s) is aconstant for each of the plurality of design widths (w₀, w⁻¹, w₊₁, etc.)and for the given direction.

According to an embodiment of the method including one or moreadditional process steps:

-   -   one or more of the targets comprises at least a first and a        second set of diffraction gratings, each set being formed of two        grating marks arranged adjacently in the same direction and in a        repeated manner, at least one of the grating marks in each set        being an asymmetric mark as defined in the preceding claims,    -   the asymmetric mark of the first grating set comprising features        of design dimension w₀−Δw₀, wherein Δw₀ is a predefined offset        value,    -   the asymmetric mark of the second grating set comprises features        of design dimension w₀+Δw₀,    -   δS and α are calculated from the equations:

α=(ΔI ₁ −ΔI ₂)/2κΔw ₀

δS=(ΔI ₁ +ΔI ₂)/2κ

-   -   wherein:        -   ΔI₁ and ΔI₂ are the differences between the plus and minus            first order diffracted intensity measured on the first and            second set of mark features obtained by the lithography step            or the etch step, on the basis of the first and second            grating set,        -   κ is a diffraction factor.

According to an embodiment, κ is calculated from a measurement of theplus and minus diffraction intensities of an additional set of twogratings located within or in the vicinity of the pattern design, eachgrating set formed of two marks arranged adjacently and in a repeatedmanner, at least one of the marks being an asymmetric mark, and whereinthe distance between two adjacent marks A′ and B′ is different in thefirst grating compared to the second grating, the difference between thedistances being pre-defined.

The method may further comprise for each target and for the initialprocess step and for at least one of the dimensions:

-   -   Calculating from δS the shift δCD of the critical dimension CD        of features of the nominal design dimension w₀, with respect to        the value of the critical dimension CD_(p) at the process        operating point for the initial process, and wherein δCD is        calculated as β.δw₀, wherein β is a proportionality factor that        expresses a linear relation between CD and the design dimension        of the features, i.e. dCD/dw=β,    -   evaluating δCD with respect to a tolerance.

According to an embodiment of the method including one or moreadditional process steps, the factor β is calculated as (α/G) wherein Gis proportional to L/p, with L the width in the direction of asymmetryof the periodic portion of the mark pattern or patterns and p thenominal pitch of the line features of the periodic portion.

The method including one or more additional process steps may furthercomprise the step of evaluating |β−1| with respect to a tolerance.

According to an embodiment of the method including one or moreadditional process steps, one or more of the targets comprises one ormore asymmetric marks configured so that the position of the centroid ofthe printed and etched asymmetric mark patterns is detectable by anoptical tool, so that the parameter S can be measured with the help ofthe tool, and wherein at least one target comprises a mark having atleast one of the design dimensions equal to the nominal value, andwherein the method further comprises the steps of:

-   -   Calculating from δS the shift δCD of the critical dimension CD        of features of the nominal design dimension w₀, with respect to        the value of the critical dimension CD_(p) at the process        operating point for the initial or the additional process step,        and wherein δCD is calculated as δS/G, wherein G is proportional        to L/p with L the width of the periodic portion of the mark and        p the pitch of the array of features in the direction of the        dimension,    -   evaluating δCD with respect to a tolerance.

According to an embodiment of the method including one or moreadditional process steps, the one or more asymmetric marks in at leastone of the targets are comb-shaped marks, wherein the periodic portioncomprises an array of parallel features extending in the direction inwhich the uniform and periodic portion are arranged.

According to an embodiment of the method including one or moreadditional process steps, the one or more asymmetric marks in at leastone of the targets are rail-shaped marks, wherein the periodic portioncomprises an array of parallel features extending in the directionperpendicular to the direction in which the uniform and the periodicportion are arranged.

According to an embodiment of the method including one or moreadditional process steps, at least one of the targets comprises a firstgroup of asymmetric marks of which the uniform and periodic portions arearranged in a first direction, and a second group of the same asymmetricmarks, of which the uniform and periodic portion are arranged in asecond direction perpendicular to the first, and wherein δS isdetermined for each of the two directions.

According to an embodiment of the method including one or moreadditional process steps, one or more of the targets comprisesasymmetric marks configured so that the position of the centroid of theasymmetric mark patterns is detectable by an optical tool, so that theparameter S can be measured with the help of the tool, and wherein themarks are arranged in cross-shapes, each cross comprising four identicalmarks.

The target used in the method including one or more additional processsteps may comprise at least one first area and at least one second area,the marks in the second area being mirrored with respect to the marks inthe first area.

According to an embodiment, the initial process step is a lithographystep using a lithographic mask or an etch step following the lithographystep and wherein the one or more targets are included in thelithographic mask.

According to an embodiment, the initial process step is a step ofproducing the device design on the lithographic mask.

The disclosure is equally related to a metrology mark applicable in themethod according to the disclosure.

The disclosure is equally related to a method for determining an edgeplacement error between two features of two respective patterned layersof a semiconductor chip, comprising the steps of:

-   -   determining the dimensions of the first and second feature by        the method of the disclosure,    -   determining the overlay error between the first and second        layer,    -   determining the edge placement error on the basis of the overlay        error, taking into account the dimensions of the first and        second feature as determined in the first step.

According to an embodiment, the pattern designs for producing the twolayers comprise respective parts of a hybrid target, the first partcomprising marks provided in the first pattern design, the second partcomprising marks provided in the second pattern design, and wherein themeasurement of the overlay error between the first and second layer isobtained from an overlay value measured between printed and/or etchedmark features resulting from the first and second parts.

The disclosure is equally related to a computer-implemented monitoringunit configured to be integrated in an apparatus for semiconductorprocessing including lithography and etching steps, the apparatusfurther comprising metrology tool, wherein the monitoring unit isconfigured to execute the steps of the method according to thedisclosure.

According to an embodiment, the metrology tool is an image based overlaytool or a diffraction based overlay tool. The monitoring unit mayfurthermore be configured to calculate—on the basis of the results ofthe evaluation—updated processing parameters of one or more processesperformed by the semiconductor processing apparatus, and feedback theupdated parameters to the apparatus. The monitoring unit may comprise amemory provided with a computer program for executing the method steps,when the program is run on the monitoring unit.

The disclosure is equally related to a computer program productapplicable in the monitoring unit of the disclosure, and configured toexecute the method steps, when the program is run on the monitoringunit.

The disclosure is furthermore related to a method for verifying pitchwalk errors during the manufacturing process of arrays of semiconductordevice features by self-aligned multiple patterning, the manufacturingprocess including the steps of producing—by lithography and etching—amandrel structure that serves as a starting point for the self-alignedmultiple patterning, wherein the lithography steps include the exposureof a resist layer through a lithographic mask, wherein the lithographicmask comprises one or more metrology targets, each target comprising oneor more comb-shaped metrology marks, designed to produce mandrelstructures comprising a base portion and a periodic portion, theperiodic portion comprising a mandrel array of parallel features ofequal width extending outward from the base portion, the arrays of themandrel structures having the same pitch p, and one or more differentwidths of the parallel features, and wherein the structures obtained byself-aligned multiple patterning on the basis of the mandrel structuresare serpentine-shaped structures comprising arrays of increasedperiodicity at each self-aligned patterning step, with the position ofthe centroid of the serpentine-shaped structures in a directiontransverse to the periodicity of the arrays being sensitive to the widthof the parallel features, and wherein the method comprises determiningthe positions of the centroids and/or the positions of the mandrelstructures and evaluating the positions with respect to a condition ofno or essentially no pitch walk errors occurring in one or more of thearrays of semiconductor device features produced in the manufacturingprocess.

According to example embodiments of the method for verifying pitch walkerrors, the lithographic mask comprises one or more metrology targets,each target comprising at least a first and second comb-shaped metrologymark, wherein the marks are designed to produce a first and second “asdesigned” mandrel structure comprising a base portion and a periodicportion, the periodic portion comprising an array of parallel featuresextending outward from the base portion, and wherein according to the“as-designed” mandrel structures:

-   -   the arrays of both mandrel structures have the same pitch p,    -   the first mandrel structure has features of width w₀−Δw,    -   the second mandrel structure has features of width w₀+Δw, with        w₀ the nominal value of the feature width and p the nominal        value of the array pitch used for obtaining a uniform        periodicity in self-aligned structures produced on the basis of        the mandrel structures, and with Δw a predefined deviation from        the nominal width,    -   the mask comprises mandrel patterns configured for the        production of arrays of device features associated to the one or        more metrology targets, the associated arrays being produced by        self-aligned multiple patterning on the basis of the mandrel        patterns, which are designed to produce mandrel structures        having features of width essentially equal to or similar to w₀        and pitch essentially equal to or similar to p        and wherein the method comprises the steps of:    -   producing first and second “real” mandrel structures by        lithography and etching, on the basis of the first and second        comb-shaped mark. It is understood that printed versions of the        mandrel structures are obtained after lithography (structures        with resist material still present) after which the etched        versions of the structures are obtained by the etching process        (e.g., after resist removal). This means that measurements can        be done on the mandrel structures after lithography as well as        after etch,    -   producing serpentine-shaped structures by one or more        self-aligned patterning steps, on the basis of the first and        second mandrel structures, the serpentine-shaped structures        comprising arrays of increased periodicity at each patterning        step,    -   for the lithography and/or the etch step, determining the        positions of the centroids of the first and second mandrel        structures in a direction transversal to the mandrel arrays,        and/or: for one or more of the self-aligned patterning steps,        determining the positions of the centroids of the        serpentine-shaped structures, in a direction transversal to the        increased periodicity arrays.    -   evaluating the positions with respect to a condition of no or        essentially no pitch walk errors occurring in the mandrel arrays        and/or the serpentine structures.

According to an embodiment of the method for verifying pitch walkerrors, each target further comprises one or more offset marks designedto produce, by the lithography and etch steps and by the self-alignedpatterning steps, one or more offset structures, and wherein thepositions of the centroids are determined with respect to acharacteristic position of one or more of the offset structures.

According to an embodiment of the method for verifying pitch walkerrors, each target comprises:

-   -   a first pair of marks consisting of the first comb-shaped mark        and an offset mark placed at a nominal distance therefrom,    -   a second pair of marks consisting of the second comb-shaped mark        and an offset mark placed at the nominal distance therefrom,        wherein the centroid positions are determined as the distances        S(+Δw) and S(−Δw) between the centroids of the mandrel        structures and/or the serpentine structures produced on the        basis of the first and second mark, and a characteristic        position of a structure produced on the basis of their        respective offset marks.

The offset marks of the first and second pair of marks may be the mirrorimages of the first and second marks respectively, where the mirrorimages are obtained by mirroring the first and second mark about an axisparallel to the arrays, wherein the characteristic positions are thepositions of the centroids of the mandrel structures and/or theserpentine structures produced on the basis of the offset marks.

The offset marks of the first and second pair of marks may be segmentedsymmetrical marks, wherein the characteristic positions are thepositions of the centroids of array structures produced on the basis ofthe symmetrical segmented offset marks.

According to an embodiment of the method for verifying pitch walkerrors, the mandrel patterns configured for the production of arrays ofdevice features associated to the one or more metrology targets comprisea reference mark for each of the metrology targets, the reference markbeing designed to produce a mandrel structure having a uniform portionand a periodic portion which have the same dimensions transversal to themandrel structure as the mandrel structures for which the first andsecond mark are designed, the reference mark being designed forproducing a mandrel array with pitch equal to p and feature width equalto w₀.

The reference mark for each metrology target may be included in therespective target. The mask may comprise an offset reference mark foreach reference mark, the offset reference mark being designed toproduce, by the lithography and etch steps and by the self-alignedpatterning steps, a reference offset structure, wherein the position ofthe centroid of the offset structure can be determined with respect to acharacteristic position of the reference offset structure.

According to an embodiment of the method for verifying pitch walkerrors, the evaluation step comprises:

-   -   Calculating for each self-aligned patterning step, a parameters

${{\delta \; S} = {\frac{{S\left( {{+ \Delta}\; w} \right)} + {S\left( {{- \Delta}\; w} \right)}}{2} - S_{0}}},$

wherein S₀ is a reference value for the distance S, determined for amandrel structure or a serpentine structure obtained from a comb markwith the same pitch p as the first and second mark and designed toproduce a mandrel structure with nominal feature widths w₀, thereference value being representative of the condition of no oressentially no pitch walk errors,

-   -   Comparing δS to a tolerance, thereby evaluating a degree of        pitch walk in arrays of semiconductor features produced in the        manufacturing process.

The method of the method for verifying pitch walk errors may furthercomprise:

-   -   Calculating for each self-aligned patterning step, a parameter

${\alpha = \frac{{S\left( {{+ \Delta}\; w} \right)} - {S\left( {{- \Delta}\; w} \right)}}{2\Delta \; w}},$

-   -   Verifying the degree to which α deviates from a pre-defined        constant value α₀.

According to an embodiment of the method for verifying pitch walkerrors, the method further comprises a step of adjusting—as a functionof the result of the evaluation step—one or more processing parametersapplied in the manufacturing process and/or parameters applied in methodfor determining the centroid positions, to thereby control pitch walkduring the manufacturing process of one or more semiconductor chips.

According to an embodiment of the method for verifying pitch walkerrors, the target comprises multiple sets of comb-shaped marks,arranged in two mutually orthogonal directions.

The disclosure is equally related to a metrology target applicable inthe method for verifying pitch walk errors according to any one of thepreceding claims.

The disclosure is equally related to a computer-implemented pitch walkverification unit configured to be integrated in an apparatus forsemiconductor processing including lithography, etching and self-alignedpatterning steps, the apparatus further comprising a metrology tool,wherein the verification unit is configured to execute the followingsteps of the method for verifying pitch walk errors: determining theposition of the centroids through the control of the metrology tool, andevaluating the positions with respect to a condition of no oressentially no pitch walk errors.

In a verification unit according to the disclosure, the metrology toolmay be an image based overlay tool. The verification unit of thedisclosure may furthermore be configured to calculate—on the basis ofthe results of the evaluation—updated processing parameters of one ormore processes performed by the semiconductor processing apparatus, andfeedback the updated parameters to the apparatus.

According to an embodiment, the verification comprises a memory providedwith a computer program for executing the method steps, when the programis run on the verification unit.

The disclosure is equally related to a computer program productapplicable in the verification unit of the disclosure, and configured toexecute the method steps for verifying pitch walk errors, when theprogram is run on the verification unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an asymmetric metrology mark that is applicable inexample methods, according to example embodiments.

FIG. 1B illustrates an asymmetric metrology mark that is applicable inexample methods, according to example embodiments.

FIG. 2 illustrates the centroid position of an asymmetric markapplicable in example methods, according to example embodiments.

FIG. 3A illustrates how the centroid of an asymmetric mark may bedetermined by an optical metrology tool, according to exampleembodiments.

FIG. 3B illustrates how the centroid of an asymmetric mark may bedetermined by an optical metrology tool, according to exampleembodiments.

FIG. 4 illustrates a possible layout of a metrology target applicable inexample methods, according to example embodiments.

FIG. 5 illustrates a graph showing the relation between feature widthsof an asymmetric mark and a parameter S that is representative of thecentroid position, according to example embodiments.

FIG. 6 illustrates another metrology target applicable in examplemethods, according to example embodiments.

FIG. 7A show correlation graphs obtained from measurements on a targethaving a layout as shown in FIG. 6, according to example embodiments.

FIG. 7B show correlation graphs obtained from measurements on a targethaving a layout as shown in FIG. 6, according to example embodiments.

FIG. 8 illustrates the position of the geometrical centroid of acomb-mark applicable in example methods, wherein the line features havedesign width equal to a nominal value w₀, according to exampleembodiments.

FIG. 9 illustrates two mark patterns obtained by printing or etching onthe basis of a mark of the type shown in FIG. 8, wherein the linefeatures have deliberately shifted design widths on either side of thenominal value w₀, according to example embodiments.

FIG. 10 illustrates another example of asymmetric mark applicable inexample methods, wherein the periodic portion of the mark hasperiodicity in two orthogonal directions, according to exampleembodiments.

FIG. 11 illustrates two marks of the form shown in FIG. 10, but whereinthe design width, in one of the orthogonal directions, of theperiodically repeating features is deliberately shifted with respect toa nominal value on either side of the nominal value, according toexample embodiments.

FIG. 12 illustrates a printed or etched set of mark patterns obtainedfrom the mark design of FIG. 11, according to example embodiments.

FIG. 13 illustrates the principle of measuring overlay errors on thebasis of diffraction-based overlay targets.

FIG. 14A illustrates a diffraction grating suitable for use in examplemethods, according to example embodiments.

FIG. 14B illustrates a diffraction grating suitable for use in examplemethods, according to example embodiments.

FIG. 15A illustrates a target comprising diffraction-based marks,applicable in example methods, comprising marks with line featureshaving widths shifted with respect to the nominal value w₀, according toexample embodiments.

FIG. 15B illustrates marks with line features having widths shifted withrespect to the nominal value w₀, according to example embodiments.

FIG. 16 illustrates a target comprising diffraction-based marks, whereinthe design width of the line features is equal to the nominal width w₀,according to example embodiments.

FIG. 17 illustrates an auxiliary target applicable for determining adiffraction factor κ, according to example embodiments.

FIG. 18 illustrates a diffraction-based target applicable in examplemethods for evaluating critical dimensions in two orthogonal directions,according to example embodiments.

FIG. 19A illustrates design dimensions and printed or etched dimensionsof two layers of line-shaped features, according to example embodiments.

FIG. 19B illustrates design dimensions and printed or etched dimensionsof two layers of dot-shaped features, according to example embodiments.

FIG. 20A illustrates details of FIG. 19A, including the edge placementerror (EPE), according to example embodiments.

FIG. 20B illustrates details of FIG. 19B, including the edge placementerror (EPE), according to example embodiments.

FIG. 21 shows an example of a hybrid target, suitable for determining anedge placement error, according to example embodiments.

FIG. 22 is an image of a metrology mark that is applicable in examplemethods for monitoring pitch walk errors, according to exampleembodiments.

FIG. 23A illustrates a step in a self-aligned multiple patterningprocess applied on a structure obtained through a metrology mark of thetype shown in FIG. 22, according to example embodiments.

FIG. 23B illustrates a step in a self-aligned multiple patterningprocess applied on a structure obtained through a metrology mark of thetype shown in FIG. 22, according to example embodiments.

FIG. 23C illustrates a step in a self-aligned multiple patterningprocess applied on a structure obtained through a metrology mark of thetype shown in FIG. 22, according to example embodiments.

FIG. 23D illustrates a step in a self-aligned multiple patterningprocess applied on a structure obtained through a metrology mark of thetype shown in FIG. 22, according to example embodiments.

FIG. 23E illustrates a step in a self-aligned multiple patterningprocess applied on a structure obtained through a metrology mark of thetype shown in FIG. 22, according to example embodiments.

FIG. 24 illustrates the intensity profiles corresponding to the variousstages of the SAxP process steps shown in FIGS. 23A-23E, according toexample embodiments.

FIG. 25 illustrates an example of a target that is applicable in examplemethods, wherein the target comprising two different marks, according toexample embodiments.

FIG. 26 illustrates the different intensity profiles in images ofmandrel structures obtained via the marks of the pattern shown in FIG.25, according to example embodiments.

FIG. 27 illustrates the intensity profiles of the spacer structuresobtained after a first self-aligned patterning step, according toexample embodiments.

FIG. 28 illustrates the intensity profiles of the spacer structuresobtained after a second self-aligned patterning step, according toexample embodiments.

FIG. 29A illustrates the position-related parameters obtained fromstructures produced by lithography and/or etch on the basis of the marksin the target of FIG. 25, according to example embodiments.

FIG. 29B illustrates the position-related parameters obtained fromstructures produced by self-aligned patterning on the basis of the marksin the target of FIG. 25, according to example embodiments.

FIG. 30 illustrates a target applicable in a calibration procedure fordetermining a reference value of the position-related parameter,according to example embodiments.

FIG. 31 illustrates SEM images of various SA2-produced serpentine-shapedstructures with varying values of the feature design width w, accordingto example embodiments.

FIG. 32 illustrates a graph relating the w-values in FIG. 31 to theposition-related parameters measured on the structures of FIG. 31,according to example embodiments.

FIG. 33 illustrates an alternative target design applicable in examplesmethods, according to example embodiments.

DETAILED DESCRIPTION

According to example embodiments, the lithographic mask used for theproduction of a patterned layer in a semiconductor manufacturing processis provided with one or more metrology targets comprising one or moreasymmetric marks of a specific geometry. Within the present context, a“mark” is defined as a predefined pattern designed for metrologypurposes. A target comprises one or more marks and is included inside orin the vicinity of the area of the mask that contains the actual patternthat is to be printed and etched to form one layer of a semiconductorchip. Typical examples of asymmetric marks which are applicable inexample methods are shown in FIG. 1, which illustrates marks of the combtype (FIG. 1A) and of the rail type (FIG. 1B). Asymmetric marks appliedin example methods may be design rule compatible, meaning that the marksare dimensioned and designed so that fine-pitched rectilinear features(also referred to as line features) of the printed patterns, such as thefine-pitched legs of the comb-type marks, can be transferred to a resistlayer, so that the fine-pitched pattern can further be transferred to anunderlying layer, by an etch process.

Example methods are however not limited to metrology performed onpatterns obtained by lithographic printing and etching, but may beapplied to techniques which do not include a mask. The method may alsobe applied to patterns obtained by process steps applied after theinitial lithography and etch, e.g. deposition, further etch and polishsteps performed in double patterning applications, and including aprocess for producing the lithographic mask itself. The presentdescription will focus first on the initial lithography and etch stepsfor producing a pattern on a substrate, as a way of explaining examplemethods. The application to other process steps will be describedfurther in this specification.

As illustrated in FIG. 2, an asymmetric mark 1 that is applicable inexample methods comprises a uniform portion 2 and a periodic portion 3.The width K of the uniform portion is not necessarily equal to the widthL of the periodic portion. The mark is reproduced on a semiconductorwafer after lithography and after etching in the form of printed andetched mark patterns respectively. FIG. 2 distinguishes between the“geometric midpoint,” defined as the x location M equidistant from themark design outer edges 21, and the geometric centroid C of the pattern.The geometric centroid C is defined as the location at which theintegrated area of the pattern in the direction of asymmetry (thex-direction in this case) is half the total integrated area. In otherwords, the area covered by pattern features to the left and right of thegeometric centroid C is the same. The geometric centroid position C of amark pattern is sensitive to the width of the features in a periodicarray of features that defines the periodic portion 3 of an asymmetricmark pattern. The periodic array may be an array of comb leg features orrail features of width w and pitch p, as illustrated in FIG. 1. For agiven constant value of the pitch p of the array, narrower featurescover less area than broader features, which translates into a shift inthe centroid position.

It has been determined that a linear relationship exists between thecentroid shift and a shift in the feature width w, when the designdimensions K and L are maintained. On the basis of this finding, amethod was developed which allows the monitoring and determination ofcritical dimensions during the manufacturing process. According to anembodiment involving comb-shaped marks as shown in FIG. 1A and FIG. 22,the method is applied for monitoring the occurrence of pitch walk errorsin arrays of features produced by a multiple patterning process. Thisembodiment is described later with reference to FIGS. 22 to 33. First, amethod is described for determining the centroid shift of a nano-scaledmark pattern, by determining a position that coincides with or isrepresentative of the position of the geometric centroid C as definedabove. This representative centroid position can be determined by anoptical metrology tool, capable of capturing an image of the markpattern. Such a tool is known per se in the art. In example methods, theoptical metrology tool is configured so that the pitch of the printedand etched arrays (the comb legs or rails in FIG. 1) is not resolvableby the tool. FIGS. 3A and 3B illustrate the images 7 and 8 as seen bythe optical metrology tool, of a symmetric mark pattern 5 (in this casea solid rectangle) and an asymmetric comb-type mark pattern 6. Thegeometric midpoints of the patterns are indicated by centerlines M. Thetool views the patterns against a contrasting background; in the case ofFIG. 3, the patterns are shown as dark fields, set against a lightbackground (but the opposite is also possible depending on the relativereflectivity of the patterned versus unpatterned areas in the metrologywavelength band). The optical tool allows to measure the intensityprofiles 9 and 10 as a function of the dimension x transversal to themark patterns. For the symmetric mark pattern 5, the intensity profile 9is equally symmetric. For the asymmetric mark pattern 6, as the finepitch p is not resolvable by the optical tool, the tool sees anasymmetric intensity profile 10 across the mark. The optical tool thenallows to detect the position of the image centroids 11 and 12 of theprofiles. The image centroid is defined as the x location at which thex-direction-integrated intensity profile is half the total integratedintensity, i.e. the area under the profile is the same left and right ofthe image centroid (the image intensity integration is referenced to thelevel I_(ref) in the case of FIG. 3). These image centroid positionscoincide with or are representative of the geometric centroids of themark patterns (i.e. a shift in the geometric centroid translates into anequal or proportional shift in the centroid of the profiles). Within thecontext of this description and in the appended claims, unlessspecifically stated, any reference to the “position of the centroid of amark pattern” may refer to the position of the geometrical centroid orto any position representative thereof, such as the image centroid, asdescribed above. The figures illustrate that the image centroid 12 ofthe asymmetric mark pattern 6 is shifted with respect to the geometricmidpoint M of the pattern, whereas the image centroid 11 of thesymmetric mark 5 coincides with its geometric midpoint M. According toexample embodiments, the optical metrology tool that is used is an imagebased overlay tool (IBO), known per se in the art for the measurement ofoverlay errors between subsequent layers in a lithography/etch-process.This is especially useful as the IBO tool is equipped for measuring adistance between the centroids of two different mark patterns. Theusefulness of such a feature will become apparent in the followingdescription of target designs applicable in example method.

FIG. 4 shows an example of a target that is applicable in examplemethods in combination with the use of an optical tool for capturing animage of a mark pattern, as described above (e.g. an IBO tool). Thetarget comprises three pairs 15, 16 and 17 of comb-shaped marks, eachpair comprising two identical but mirrored marks. The middle pair 16 aredesigned for printing and etching an array of features of nominal pitchp. The design width w₀ is the nominal design width of the line featuresof the asymmetric mark. The value w₀ is representative of the criticaldimension of line features of a device or test pattern that is printedand etched through the mask in which the target is incorporated, as wellas processed further by additional process steps such as deposition,etch, etc. The term “representative of” is explained as follows. In eachprocess step to which example methods can be applied, line features areproduced which are characterized by their line width referred to as thecritical dimension CD. For each step, a process value CD_(p) for thecritical dimension of pattern features is determined during the patternand process development stage, involving pattern design and calibrationmethods known in the art. The result of these development procedures is(for each process step): a set of process conditions referred to as theprocess operating point, and the value of CD_(p) for a variety ofpattern features. The aim of metrology methods is to determine to whatdegree the CD_(p) values are actually obtained on the wafer. The markdesign value w₀ is chosen such that the variation of w₀ within a givenrange around the design value leads to a measurable parameter (e.g. S,see further) obtained from the metrology target, which can be used todetermine the deviation from CD_(p) in each process step. This is themeaning of “w₀ is representative of critical dimensions of features of adevice pattern”. The value w₀ could be equal to the process value CD_(p)in one or more process steps, but this is not a requirement and w₀ maydiffer (significantly) from CD_(p).

As stated, the process operating point is determined for each processstep, by calibration procedures known in the art. For lithography andetch steps, the process operating point is defined by an optimized dose,focus and etch parameters. During the calibration, the patterns definedby the lithography mask, i.e. the device patterns and the targetpatterns (for example targets as shown in FIG. 4) included in the maskare printed through a range of processing parameters, including at leastthe focus and dose of the lithographic tool. The focus-parameter is thedeviation from a pre-defined zero-defocus setting on the exposure tool,expressed for example in nm. Dose is defined as the energy appliedthrough the mask during exposure, expressed for example in mJ/cm². Bothfocus and dose are values that can be set on the lithography tool. Thecalibration process may be performed on a test wafer, on which the maskpatterns are printed multiple times, each time with an adjustment ofdose or focus. Such procedures are known in the art for the calibrationof a lithography process. The test wafer is known as a focus-energymodulation (FEM) wafer. The device patterns and marks are measured bytraditional measurement techniques such as SEM or AFM to determine theas-printed values of the feature widths (the CD of the device patternsand the w of the marks). For process steps other than lithography andetch, the process operating point is determined in the same way.

The process operating point for a process step that follows one or morepreceding steps includes optimized conditions of all the precedingsteps. For example, the process operating point of the SA1 step in aself-aligned multiple patterning process is defined by a set ofoptimized operating conditions for the lithography and etch steps forproducing a mandrel structure, and optimized conditions for the spacerdeposition following the production of the mandrel, and for the etchingfor removal of the mandrel.

Returning to the target design of FIG. 4, the upper pair of marks 15comprise an array of features of design width w_(—1)=w₀−Δw₀ and pitch p.The lower pair 17 comprise an array of features of design widthw₊₁=w₀+Δw₀ and pitch p. All pairs are designed to reproduce the twomirrored patterns at a mutual distance D_(s) with respect to each other,D_(s) being measured between the lines that separate the uniformportions and the periodic portions of the marks. Δw₀ is a predefinedoffset value for the design width w₀.

The following steps of the method according to a first embodiment aredescribed on the basis of printed versions of the marks of FIG. 4, i.e.before etching. The same method steps may however be performed on thebasis of the etched patterns or patterns obtained by other process stepsapart from lithography and etching. In FIG. 4, the detected centroidpositions (e.g., by the optical IBO tool as described above) areindicated for the printed mark patterns of the three pairs of marks. Aparameter S is defined as D−D_(s), wherein D is the distance between thecentroids of the image of the two mirrored marks, and D_(s) is theabove-described design distance between the geometric midpoints of themarks. FIG. 4 shows an embodiment in which the geometric midpoints ofthe asymmetric marks lie at the boundary between the uniform andperiodic areas of the mark (implying K=L), but this is not required, asis shown for mark 6 in FIG. 3. D_(s) is a constant determined by themark design and the relative position of marks within a target design. Sis a parameter that is representative of the centroid position of eitherof the mirrored marks of each pair, measured in the x-direction. A shiftin the centroid results in a change in the parameter S. Other choicesfor the parameter S are possible, for example the distance D itself canbe used as such a parameter, but in any case S is a linear function ofthe centroid shift measured in the x-direction. The centroid shift, andthereby S can be measured by an IBO tool, in the manner described above,by detecting the centroids of the observed intensity profiles of theimages of two mirrored marks and measuring their mutual distance in thex-direction. For the printed patterns of the three mark pairs, themeasurement yields three values, labelled S₀, S⁻¹ and S₊₁:

S ⁻¹ =D _(—1) −D _(s)

S ₀ =D ₀ −D _(s)   (1)

S ₊₁ =D ₊₁ −D _(s)

The relation between the S-values and the design values of the featurewidth w is then determined. For comb-shaped and rail-shaped asymmetricmarks, this relationship is approximately linear provided the designdimensions K and L are maintained for the comb and rail arrays. Asillustrated in FIG. 5, the data points can be approximated by abest-fitting linear function:

S=α(w−w ₀)+b   (2)

The linear-fitting may be done by any optimization method known in theart, for example by a least-squares optimization algorithm. The resultof this procedure is the parameter α. The slope α represents the rate ofchange of S as a function of the design width w, i.e.

$\begin{matrix}{\alpha = \frac{dS}{dw}} & (3)\end{matrix}$

Following this, a parameter δw₀ is calculated as:

δw ₀ =δS/α=(S ₀ −S _(0ref))/60   (4)

S_(0ref) is the S-parameter determined for a printed mark pattern at theprocess operating point for the lithography step, see above.

The value δw₀ obtained from equation (4) is the effective shift of thedesign width represented by the printed target, with respect to theprint under the reference conditions of the process operating point.When δw₀ is different from zero, this means that the device pattern hasbeen printed as if the design width of the mark features has shiftedover the value δw₀. Therefore, it does not necessarily mean that thecritical dimension of the printed features of design width w₀ hasshifted over this value. The determination of δw₀ is however anindicator for deviations from the desired on-wafer dimensions. δw₀ maythus be compared to a tolerance, thereby allowing to make an assessmentof the print quality of line features produced by the lithography step.

In some embodiments, a plurality of targets are included in thelithography mask. According to the first embodiment, each targetcomprises a plurality of asymmetric marks having a uniform portion and aperiodic portion. In some embodiments, comb-type marks and/or rail-typemarks are used. Each target may include asymmetric marks characterizedby different design widths w of the features (legs or rails), forexample, including a mark with design width of the features equal to anominal value w₀ and further including a number of design widths in arange around the w₀-value. The various widths may not necessarily beshifted by fixed amounts from w₀ (like in the previous example: shiftsof −Δw₀ and +Δw₀), but they might be distributed across a given rangethat corresponds to an expected error margin for the critical dimensionsof features produced in the various steps of the manufacturing process.FIG. 6 shows another example of a target layout that is applicable inexample embodiments. The target comprises comb-type marks arranged incross-shaped layouts. The numbers next to each cross indicate therespective design widths in nm of the comb legs of the marks thatconstitute the cross-shapes. The w₀ value is 44 nm and the design pitchp in each array of features is 96 nm. The target comprises two areasmarked by “0” and two areas marked by “180”. This indicates that marksfrom the “0” areas are mirrored with respect to marks from the “180”areas. The asymmetric marks are arranged around a central mark 20 thatserves as a positioning reference for the imaging tool used to measurethe centroid shifts. As stated above, this may be an IBO tool which isequipped to measure the distance D between one of the mark centroids ina “0” area with respect to the corresponding mark centroid in a “180”area. The arrangement of the marks in a cross-design allows to measure Din two orthogonal directions x and y, i.e. to obtain S_(x)=D_(x)−D_(s)and S_(y)=D_(y)−D_(s) for each of the design widths. The S₀-value (in xand y) used for equation (4) is measured on the mark pair designed forline widths equal to the nominal w₀ (see equations (1)). The S_(0ref)value is obtained for the same mark pair, under the conditions definedby the process operating point. When the target does not comprise a markwith line features designed for the nominal width w₀ but only marksdesigned for widths in a range around w₀, the S₀ and S_(0ref) values maybe obtained from the intersection of the best-fitting linear curve andthe y-axis.

FIG. 7A shows S_(x) and S_(y) as a function of (w−w₀) for the target ofFIG. 6, as measured during the calibration procedure, at the processoperating point, therefore at the reference dose and focus conditionsfor the lithography step. The graph includes the numerical linearapproximations of the data as well as the square of the correlationcoefficient R², which is a measure for the linear relationship betweenthe x and y values (values close to 1 indicate excellent linearity),confirming the fact that the relation between S and the design widths isapproximately linear for S_(x) as well as for S_(y). From the best fitof the data points, the proportionality factor α₀ can be derived, in x-and y direction:

α_(0x)=6.7

α_(0y)=6

The suffix “0” indicates that these α-values are obtained at the processoperating point. The calibration value S_(0ref) is found as the S-valuefor w=w₀. In this case, S_(0ref)≈˜185 nm in x and y direction. FIG. 7Billustrates the same graph for a rail-type mark, designed also for w₀=44nm and p=96 nm. The linear relation is again apparent.

The α₀-values themselves can be used for verifying the stability of themanufacturing process and the metrology process, when example methodsare performed. In practice, a plurality of n targets, for exampletargets of the type shown in FIG. 6, may be distributed between andaround the manufactured device patterns printed with the lithographymask into which the targets are incorporated. Each of the n targetsyields a number of S-values (in x and y) for each of the design widthsincluded in the target (e.g. w=36 nm, 40 nm, 44 nm, 48 nm, 52 nm, 96nm). For w=96 nm the mark is just a symmetric rectangle and no shift isdetected. For each target, the best fit of the data points yields avalue for α, resulting in a series of values α_(i)(I=1 . . . n).Deviations from α₀ indicate a process instability or an instability inthe metrology process. A mask may also comprise targets of differentdesigns, for example designed for different w₀ and/or p values. In thatcase each target design results in S and α values which are to beassessed with respect to specific values of S_(0ref) and α₀.

To summarize: on the basis of the latter example of n targets of thedesign of FIG. 6, distributed across the field of view of thelithography mask, the following steps are performed according to thefirst embodiment:

-   -   The lithographic printing step is performed, i.e. the mask is        exposed to a light source and the mask patterns are reproduced        on a resist layer, resulting in visible mark patterns on the        resist layer, with nano-scaled dimensions,    -   For each target i (i=1 . . . n), the values S_(x) and S_(y) are        measured for the design widths 36 nm, 40 nm, 44 nm, 48 nm, 52        nm. Measurement may be done by an IBO tool as described above.    -   In each target, the proportionality factors α_(ix) and α_(iy)        are determined from the best linear fit of the data points in a        graph that sets out S as a function of (w−w₀),    -   The shift in the design width δw_(0x) and δw_(0y) is calculated        from equation (4), on the basis of a previous calibration from        which S_(0xref) and S_(0yref) have been obtained,    -   δw_(0x) and δw_(0y) are assessed with respect to a tolerance,    -   The values of α_(ix) and α_(iy) are compared to the values        obtained in the calibration procedure α_(0x) and α_(0y).        |α_(ix)−α_(0x)| and |α_(iy)−α_(0y)| are compared to a tolerance.

The assessment of δw_(0x) and δw_(0y) gives an indication of possibleinadmissible deviations from the critical dimensions of line features inthe printed device patterns. In some embodiments, the various targetsare placed in close proximity to critical device features for which theCD monitoring is used. The various targets may be designed for differentvalues of w₀. The monitoring of |α_(ix)−α_(0x)| and |α_(iy)−α_(0y)|gives an additional indication of possible process irregularities, aswell as instability of the metrology process as such.

The same method steps may be performed on the basis of the etchedpatterns, i.e. after resist development and etching to reproduce theactual patterns on a semiconductor wafer. This yields an additional setof values δw_(0x) and δw_(0y) (computed on the basis of specificS_(0xref) and S_(0yref) values), and |α_(ix)−α_(0x)| and|α_(iy)−α_(0y)|, to be assessed with respect to a specific tolerance.The calibration procedure for determining S_(0xref) and S_(0yref) isagain a FEM-based procedure, taking into account not only variations ofthe lithographic parameters, but also of etch parameters, such as etchtime, etch rate, voltage, chamber pressure, etc.

As stated above, the shift δw₀ in the design feature width w is notnecessarily equal to the shift of the actual critical dimension ofdevice features on the wafer which are designed to be essentially equalto w₀ and which are printed as part of or in the vicinity of a target asdescribed above and shown by example embodiments in FIGS. 4 and 6.According to a second embodiment, an estimation of the actual CD shift,hereafter referred to as δCD is determined on the basis of IBO-typetargets as described above and of which an example is shown in FIGS. 4and 6. The shift is determined with respect to the critical dimensionvalue CD_(p) obtained at the process operating point, of line featuresproduced in a process step and, designed for the nominal width value w₀or derived from such mark line features (for example line featuresobtained in a double patterning process). For variations about thisprocess operating point representative of the anticipated manufacturingvariation, the width CD of a line feature produced in the process stepcan thus be written as:

CD=CD _(p) +δCD   (5)

A factor β can furthermore be calculated so that:

δCD=β.δw ₀   (6)

The factor β represents the rate of change of CD as a function of thechange of the design width w:

$\begin{matrix}{\beta = \frac{dCD}{dw}} & (7)\end{matrix}$

The factor β is directly related to α as will be demonstrated hereafterfor the lithography step by which a mark is first reproduced on a wafer.FIG. 8 shows the design dimensions of a comb mark having a uniformportion of width K and a periodic portion of width L, measured in thedirection of asymmetry x. The periodic portion comprises comb featuresof width w₀ at pitch p. The location in the x-direction of the geometriccentroid C_(g) is given by the following formula:

$\begin{matrix}{C_{g} = {\frac{1}{2}\left( {{\frac{L}{p}w_{0}} - K} \right)}} & (8)\end{matrix}$

This follows from the condition that the surface area of the markpattern is the same to the left and right of C_(g). FIG. 9 shows twoprinted mark features obtained from two different variations of the markshown in FIG. 8. The upper mark pattern is obtained from a comb mark asin FIG. 8 but with comb features of design width w₀−Δw₀, while the lowermark pattern is obtained from a comb mark as in FIG. 8 but with combfeatures of target width w₀+Δw₀. Δw₀ is a predefined offset value. Withβ defined as in formula (6), the width of the printed comb features inFIG. 9 can thus be written as CD_(p)−βΔw₀ and CD_(p)+βΔw₀ respectively,the value CD_(p) as defined above, i.e. the critical dimension at theprocess operating point.

The position of the centroids C⁻ and C⁺ along the x-axis can be writtenon the basis of formula (6), as follows:

$\begin{matrix}{C^{-} = {\frac{1}{2}\left( {{\frac{L}{p}\left( {{CD}_{p} - {{\beta\Delta}\; w_{0}}} \right)} - K} \right)}} & (9) \\{C^{+} = {\frac{1}{2}\left( {{\frac{L}{p}\left( {{CD}_{p} + {{\beta\Delta}\; w_{0}}} \right)} - K} \right)}} & (10)\end{matrix}$

Subtracting equations (10) and (9) yields:

ΔC=C ⁺ −C ⁻=(L/p)βΔw ₀ =gβΔw ₀   (11)

With g=L/p defined as the intrinsic gain factor of the mark design. Fromthe fact that the parameter S in FIG. 4 is derived from mirrored marks,it follows that:

|ΔS|=2|ΔC|=GβΔw ₀   (12)

Where the gain of the mirrored mark pair is G=2g. Taking into accountthat α=ΔS/Δw₀ it follows that β=α/G, i.e. given the mark designs andtheir arrangement within the target, β can be calculated directly fromα.

During manufacturing, α and β can be determined at each process step(e.g. at litho, etch, polish, SA1, SA2, . . . ) allowing thedetermination and assessment of δw₀ as described above, while anestimation of the on-wafer CD can thus be calculated as CD=CD_(p)+βδw₀.

The factor β can be referred to as a patterning/metrology fidelityfactor with β equal to 1 for perfect fidelity. β embodies various knowneffects (e.g., defocus, MEEF, LER, proximity bias, aberration, etc.)that may influence pattern transfer and measurement. While β can changewith each patterning step (litho, etch, deposition, etc.), and candepend on mark orientation, particularly for anisotropic lithographyillumination, it should be relatively constant for a given set ofprocess/metrology conditions. The value of β itself may be used inexample methods for monitoring process or metrology instabilities, whichmay be flagged when |β−1| exceeds a given tolerance.

The parameter g=L/p is a constant based on the mark design and referredto here as the intrinsic mark amplification factor. In order to enhancemetrology signal to noise ratio, the marks may be designed so that g issignificantly higher than 1. This ensures for example that L iseffectively a constant in the printed and etched versions of a markpattern, so that the relation between α and β holds (and provided K isalso constant). During experiments, it was found that high values can beobtained for the proportionality factor α. This means that examplemethods reach higher levels of accuracy than existing metrology methods.

The invention is not limited to the above embodiments of targetssuitable for image-based determination of δS, α, β and δCD (such as byusing IBO tools for determining the centroid positions). Any relativemark placement measurement is always made between two or more marks.Instead of pairs of mirrored asymmetric marks, the target may comprisepairs of a symmetric mark and an asymmetric mark. The centroid of thesymmetric mark serves as a reference for the position of the centroidsof the asymmetric marks having different design values of the featurewidth w. The position-dependent parameter may then be measured as thedistance between the centroid of a symmetric mark and the centroid ofthe paired asymmetric mark. The targets could even comprise a singlesymmetric mark, for example placed centrally in the target design, andserving also as a positioning reference, wherein all the asymmetricmarks are referenced with respect to the single symmetric mark in theabove-described manner, i.e. by measuring the shifting difference of theasymmetric mark centroids with respect to the centroid of the singlesymmetric mark. All the above-described method steps and formulas remainvalid for these variations of the target and mark designs. Eachasymmetric mark is characterized by an intrinsic gain factor g=L/p;where, by definition, g=0 for symmetric marks. For mirrored mark pairsG=g₁+g₂=L₁/p₁+L₂/p₂. Thus, when one of the marks is symmetrical, G=g;whereas, when the mirrored marks are identical G=2g, as in Equation 12.

For a target comprised of asymmetric marks as described above, therelation between β and α can be written as:

β=α/G   (13)

with G the constant gain factor dependent on the mark design andarrangement. G is generally proportional to g=L/p, such as G=g or G=2g.Equation (13) is valid at least for comb-marks and rail-marks, and forprocess steps wherein the mark pattern obtained by the process stepmaintains the original shape of the marks (e.g. first lithography andetch steps, deposition step, polish step).

For any mark pattern derived from an asymmetric mark applicable inexample embodiments, an estimation of the factor β can be obtained by alinear fitting (done by any optimization method known in the art), forexample by a least-squares optimization algorithm, in an analogue way asfor the determination of α described above, i.e. based on a targetcomprising asymmetric marks with different design widths w of the linefeatures, in a range about a nominal value. In this way, the linearrelation is determined between the measured CD-values of different markpatterns on the substrate (measured for example by SEM) and the designvalues of the different mark patterns with design widths w about w₀. Theresult of this procedure represents an estimation of the parameter β.

A specific embodiment for the image-based determination of a parameter Sfollows from the following observations. From equations (4) and (6) and(13), it follows that:

δCD=βδw ₀=(α/G)(δS/α), so that

δCD=δS/G   (14)

As G is constant, the CD shift with respect to CD_(p) can thus bedetermined from one measurement of S₀, i.e. from a single measurement ofthe S-parameter on a pair of mirrored asymmetric marks comprising linefeatures of design width w₀. This represents a “basic” embodiment, wherean “IBO type” target may comprise only a single asymmetric mark type,designed for w₀ (for example a single pair of mirrored marks with linefeatures of design width w₀, i.e. the central pair 16 of FIG. 4). Themarks designed for widths in a range around w₀ are not required in thiscase. According to this embodiment however, δw₀, α and β are notdetermined and the above-described monitoring of process and metrologyinstability cannot be performed.

An example of an asymmetric mark that is suitable for use in examplemethods and that comprises an array of 2-dimensional features is shownin FIG. 10. The drawing shows the target dimensions of the mark. Themark again has a uniform portion 2 with width K in the x-direction and aperiodic portion 3 having width L in the x-direction. The periodicportion now has periodicity in two orthogonal dimensions x and y, i.e.the mark features are rectangular dots of width w_(0b) in thex-direction and height w_(0a) in the y-direction. The pitch of the markfeatures in the x and y direction is respectively p_(b) and p_(a). Theposition of the geometric centroid C_(g) can again be calculated byexpressing the condition that the surface of mark features to the leftand right of the centroid position is the same. This yields:

$\begin{matrix}{C_{g} = {\frac{1}{2}\left( {{\frac{w_{0\; a}w_{0\; b}}{p_{a}p_{b}}L} - K} \right)}} & (15)\end{matrix}$

FIG. 11 shows images of the design dimensions of two marks based on themark of FIG. 10, but with a deliberate offset of the design width w_(0b)of the mark features in the x-direction. The offset is respectively +Δw₀and −Δw₀, with Δw₀ a predefined value, with Δw₀<<w_(0b). The offset isadded or detracted at the side of the design width w_(0b) that faces thenon-periodic portion with width K of the mark (the left-hand side in thedrawings), so that the dimension L is maintained. The design widthw_(0a) in the y-direction remains unchanged. The pitch in x and ydirections also remains the same, p_(b) and p_(a) respectively. Thegeometric centroid of the marks in FIG. 11 is shifted in the x-directionwith respect to the centroid position in FIG. 10, to respective centroidpositions C_(g) ⁻ and C_(g) ⁺. These positions are expressed as:

$\begin{matrix}{C_{g}^{-} = {\frac{1}{2}\left\lbrack \left( {{g_{b}\left( {w_{0\; b} - {\Delta \; w_{0}}} \right)} - K} \right) \right\rbrack}} & (16) \\{{C_{g}^{+} = {\frac{1}{2}\left\lbrack \left( {{g_{b}\left( {w_{0\; b} + {\Delta \; w_{0}}} \right)} - K} \right) \right\rbrack}}{{{Wherein}\mspace{14mu} g_{b}} = {\frac{w_{0\; a}}{p_{a}p_{b}}L}}} & (17)\end{matrix}$

A target comprising mirrored pairs of the three marks shown in FIGS. 10and 11, and possibly comprising additional marks with offset valuesdifferent from +Δw₀ and −Δw₀, can be applied in example methods in thesame way as the target of FIG. 4. The centroid shifts can be measured byan IBO tool configured so that the pitches p_(b) and p_(a) of theprinted and etched marks are not resolvable by the tool. In this way,the centroid shifts can be detected by the tool through analysis of theimage intensity, as explained with regard to FIG. 3. This analysisresults in the δS-values (obtained from the measured S₀ and thepreviously measured S_(0ref) obtained at process setup) for the variousmark pairs, the α-value and the design width offset in the x-directionδw_(0b), in the same manner as described above. The β-value is againdirectly related to the α-value, through the amplification factor g_(b).This is explained on the basis of FIG. 12, which shows an image of themark patterns obtained when the marks of FIG. 11 are printed or etched.The critical dimensions of the mark features in the x-direction canrespectively be written as CD_(bp)−βΔw₀ and CD_(bp)+βΔw₀. Therefore, thecentroid positions C− and C+ are:

$\begin{matrix}{C-={\frac{1}{2}\left\lbrack \left( {{g_{b}\left( {{CD}_{bp} - {{\beta\Delta}\; w_{0}}} \right)} - K} \right) \right\rbrack}} & (18) \\{C+={\frac{1}{2}\left\lbrack \left( {{g_{b}\left( {{CD}_{bp} + {{\beta\Delta}\; w_{0}}} \right)} - K} \right) \right\rbrack}} & (19)\end{matrix}$

Wherein CD_(bp) is the width in the x-direction of the mark featureshaving design width w_(0b) as printed or etched under the conditions ofthe process operating point for the print or etch step. Subtractingequations (19) and (18), and applying the relationship of equation (12)then yields:

ΔC=C+−C−=ΔS/2=g _(b.) β.Δw ₀,   (20)

so that β can again be found as β=α/2g, which allows to estimate thecritical dimension in the x-direction as:

CD _(b) =CD _(bp) +β.δw _(0b)   (21)

Instead of applying targets designed for measurement using animage-based tool such as IBO tools, example methods may employ targetswhich allow a measurement based on diffraction phenomena. Thismeasurement may be done in a manner known in the art and by a tool knownin the art for measuring overlay errors between two layers, known as aDBO (diffraction based overlay) tool. FIG. 13 shows the layout of a DBOoverlay target as known in the art, formed of an arrangement ofoptically distinct elements (A,B) in an interlaced diffraction gratingconfiguration with a pitch “P”, typically on the order of 500-1000 nm.In other words, a first grating consisting of elements A is interlacedwith a second grating consisting of elements B. Within each period “P”are the repeating elements (A,B). The grating elements A and B might besufficiently different to cause a measurable difference between the plusand minus first order diffracted intensity ΔI=I₊₁−I⁻¹. This intensitydifference is measured in a DBO overlay tool. It is a measure for theoverlay value OVL which allows to determine overlay errors when theelements A and B are printed through different lithography masks.

A DBO target applicable in example embodiments comprises a similarinterlaced grating configuration, but with elements A and B included inthe same lithographic mask. In the terminology of example embodiments,elements A and B are referred to hereafter as “marks” or “gratingmarks”. In addition, at least one of the marks A or B is asymmetric inthe sense described above. FIGS. 14A and 14B show two examples of thelayout of a DBO target applicable in example embodiments. In the firstexample (FIG. 14A), mark A is symmetric and mark B is asymmetric: it isa comb mark comprising a fine structure arranged at a pitch p<<P. p islower than P to a degree that precludes diffraction induced by the finestructure itself, which could confound or detract from the diffractionfrom the grating elements at the coarse pitch P. FIG. 14B shows anexample where both A and B are asymmetric. A and B cannot be mirrorimages however: they might have dimensions sufficiently different tocause a measurable difference in the plus and minus first orderdiffracted intensity. The marks applied in at least one grating might beasymmetric in order for patterning process conditions to induce arelative movement of the centroids C of the asymmetric marks B of oneprinted and/or etched grating with respect to the centroids C′ of themarks A of the symmetric grating (FIG. 14A) or with respect to thecentroids C′ of the marks A of the opposing asymmetric grating (FIG.14B). Such relative movements cause a proportional change in therelative intensity of the plus and minus first order diffraction fromthe gratings. Thus, a DBO tool can measure a centroid shift by measuringthat intensity difference on a printed or etched or otherwise processedversion of the DBO target, formed of the grating sets formed of theprinted or etched marks or otherwise processed A and B.

A DBO target according to example embodiments comprises two sets ofinterlaced grating marks, each comprising an asymmetric mark withintentionally and oppositely shifted widths of the mark features in theperiodic portion of the mark. An example of a suitable target design isshown in FIG. 15A. The target comprises a first and second set ofgratings 30 and 31. The first grating set 30 comprises interlacedgrating marks A and B1. A is a symmetric grating mark formed of linefeatures having a design width w₀ and B1 is a comb mark with markfeatures having design width w₀−Δw₀. The second grating set 31 comprisesinterlaced grating marks A and B2. A is the same symmetric grating markas the one used for the first grating set 30 and B2 is a comb mark withmark features having design width w₀₊Δw₀, i.e. designed for printing andetching features of that width. The dimensions K and L are the same forboth asymmetric mark gratings B1 and B2. The mark A may not necessarilybe a mark with line features designed for width w0. Any symmetric markdesign is applicable for use as mark A.

FIG. 15B shows the same target design except that the widths of the combfeatures are designed to be equal to w₀. Grating mark A is the same asin FIG. 15A, grating mark B is the same as B1 and B2 except for thedifference in comb feature width. The target designs of FIG. 15A areoffset versions with respect to the design of FIG. 15B by subtractingand adding the offset value Δw₀, in the manner illustrated for IBOtargets in FIG. 4. The design of FIG. 15B is not necessarily included ina DBO target suitable for example methods. In the mark design of FIG.15B, the distance between the centroids of the grating marks A and B is0.5P.

When the gratings 30 and 31 are printed and etched, the centroids of theprinted and etched grating marks B1 and B2 will shift as a function ofthe offset value Δw₀. This is illustrated in FIG. 16 which shows theprinted or etched mark pattern obtained from the target of FIG. 15A. Thedistance between the centroids of the mark gratings A and B1 and A andB2 is chosen as the parameter S representative of the centroid positionof the grating marks. In accordance with equation (3), the distance S isa linear function of the design width, i.e. this distance isrespectively shifted for the two gratings shown in FIG. 15A by a valueof +αΔw₀ and −αΔw₀, relative to the distance for a printed or etchedtarget according to FIG. 15B, i.e. wherein the line features aredesigned for the width w₀. The latter distance is equal to 0.5P+δS,where δS is defined as in equation (4). Therefore, the respectivedistances between the centroids of the mark features in FIG. 16 are:

S ₁=0.5P+δS+αΔw ₀   (22)

S ₂=0.5P+δS−αΔw ₀   (23)

The intensity differences ΔI₁ and ΔI₂ are measured on the printed oretched versions of the two gratings 30 and 31, using a DBO tool. Theseparameters are proportional to the values δS+αΔw₀ and δS−αΔw₀ via adiffraction coefficient κ:

ΔI ₁=κ(δS+αΔw ₀)   (24)

ΔI ₂=κ(δS−αΔw ₀)   (25)

Solving δS and α from these equations leads to:

α=(ΔI ₁ −ΔI ₂)/2κΔw ₀   (26)

δS=(ΔI ₁ +ΔI ₂)/2κ  (27)

The coefficient κ can be determined from a measurement on a targetcomprising sets of gratings similar to FIG. 15A. These grating sets areshown in FIG. 17. The target comprises two sets of gratings 30′ and 31′formed by the same marks A′ and B′ but with a well-defined difference inthe distance between two adjacent marks A′ and B′. The pitch P is thesame for both sets of gratings 30′ and 31′. In grating set 30′, the ‘asdesigned’ distance between the centroids of two adjacent marks A′ and B′is 0.5P+ΔD. In grating set 31′, this distance is 0.5P−ΔD. The shift ΔDis a well-defined value that is significantly smaller than 0.5P so thatgrating sets 30′ and 31′ represent slightly shifted versions of thegrating with ΔD=0. When the target is printed or etched, the centroidposition of the asymmetric mark B′ will be shifted over a distance δ.This means that:

-   -   the distance between the printed or etched mark patterns A′ and        B′ in grating set 30 is 0.5P+ΔD+δ,    -   the distance between the printed or etched mark patterns A′ and        B′ in grating set 31 is 0.5P−ΔD+δ.

The intensity differences ΔI₁ and ΔI₂ are measured on the printed oretched versions of the two gratings, using the DBO tool. Once again,these parameters are proportional to the values ΔD+δ and −ΔD+δ via thediffraction coefficient κ:

ΔI ₁=κ(ΔD+δ)   (28)

ΔI ₂=κ(−ΔD+δ)   (29)

Solving κ and δ from these equations leads to:

κ=(ΔI ₁ −ΔI ₂)/2ΔD   (30)

δ=ΔD(ΔI ₁ +ΔI ₂)/(ΔI ₁ −ΔI ₂)   (31)

In this way, the target shown in FIG. 17 allows to calculate κ.Equations (26) and (27) then allow to calculate δS and α. The factor βcan again be calculated from α. As the parameter S is now the distancebetween a symmetric mark A and an asymmetric mark (B, B1 or B2) and notbetween two mirrored asymmetric marks, the relation between α and β is:

β=α/g with g the amplification factor L/p

Once again, β allows to estimate the critical dimension of line featuresdesigned for width w₀, from equations (5) and (6), wherein δw₀ equalsδS/α. CD_(p) is determined during a calibration procedure when theprocess operating point is determined, as described in relation to theIBO embodiment.

If a grating set as shown in FIG. 17 is not available, κ could be knownfrom specifications of the DBO tool, or from other experiments. If κ isunknown, example methods can still be performed, except that β cannot bedirectly calculated from α. From equations (26) and (27), κ can beeliminated, yielding:

δw ₀ =δS/α=(ΔI ₁ +ΔI ₂)Δw ₀/(ΔI ₁ −ΔI ₂)   (32)

This equation thus allows to calculate δw₀ obtained from equation (4)when an IBO-type target is used. The assessment of δw₀ can then be donein the same way as described above for the IBO embodiment.

The DBO-type targets of FIGS. 15A and 17 thus allow to perform examplemethods according to the first and second embodiments described above,including the assessment of |α−α₀| and of |β−1| with respect to atolerance. Also in analogy with the IBO-type targets, DBO-type targetsmay include grating sets oriented in two orthogonal directions x and y,for the assessment of the critical dimensions in x and y. FIG. 18illustrates an example of a typical DBO-type target applicable invarious embodiments. Grating sets 35 to 38 represent two pairs ofgrating sets like the one shown in FIG. 15A (i.e. including comb-markswith deliberately offset line feature widths), for determining δS_(x)and δS_(y). Gratings sets 39-42 comprise two pairs of grating sets likethe one shown in FIG. 17, for determining κ_(x) and κ_(y). Grating sets39-42 may not necessarily form a single target area together with thegrating sets 35-38, but these grating sets 39-42 could be placed inseparate target area in the vicinity of the pattern that is to beproduced, such as in the vicinity of the grating sets 35-38.

According to a further embodiment, the estimated critical dimensionsdetermined in the manner described above may be used for estimating anedge placement error (EPE) between features produced in two differentlayers. This may be achieved by applying the method as described abovein each layer based on targets having design widths which arerepresentative of the features in question, and further taking intoaccount the overlay error between the two layers.

FIG. 19A shows an example of a two-layer structure, the first layercomprising an array of parallel line-shaped features 40, and the secondcomprising an array of rectangular features 41 overlying the lines. Thistypically occurs when patterning an array of metal lines in the firstlayer and an array of contact vias in the second layer, wherein the viasmay overlap the lines in order to connect the lines to an upper level ofa 3D interconnect structure.

FIG. 19A shows the as-designed dimensions of the lines 40 and the viafeatures 41 and of the degree of overlap between the layers. FIG. 19Bshows a possible situation after the actual production steps of thelayers (i.e. lithography and etching of both layers). The lines 40 arereproduced as lines 40′ and the rectangular via features are reproducedas ellipse-shaped features 41′. FIGS. 20A and 20B show details of theimages of FIGS. 19A and 19B. The width in the y-direction of the linefeatures 40′ as printed and etched is referred to as CD1y. The width inthe y-direction of the via features 41′ is referred to as CD2y. The factthat these dimensions deviate from the as-designed values, together withan overlay error OVL between the two layers, results in the fact that toone side of the line features, the via features do not overlap theseline features. This error is referred to as the edge placement error(EPE) and as illustrated in FIG. 20B, this error is quantified as:

EPE=D ₀+OVL−(CD1y+CD2y)/2,   (33)

Wherein D₀ is the design distance between the center lines of the linefeatures 40 and the via features 41.

Example methods allows to estimate CD1y and CD2y by including an optical(e.g. IBO-based) or a diffraction (e.g. DBO based) target in each of themasks used to print and etch the layers, wherein the optical ordiffraction targets are designed on the basis of the design dimensionsof the line and contact via features 40 and 41. The mask for the firstlayer may for example comprise an IBO-type target similar to the oneshown in FIG. 4, wherein w₀ equals the design dimension of the linefeatures 40. The mask for the second layer may comprise an IBO typetarget comprising mirrored pairs of the marks illustrated in FIGS. 10and 11 wherein at least the design dimension w_(oa) in the y directionis equal to the design y-dimension of the via features 41.

CD1y and CD2y are determined by performing example methods on the twolayers. OVL is an overlay measurement between the two layers, as knownin the art and performed by an IBO tool or a DBO tool depending onwhether the marks are optical or diffraction based. From equation (33),the EPE can then be calculated. This is a straightforward way ofdetermining edge placement errors, without requiring SEM measurements orthe like.

The targets described so far can be used for determining the OVL value,for example by measuring the shift of a pair of mirrored mark patternsobtained from the target shown in FIG. 4, the shift being measuredbetween the version of the mark pair in the first layer with respect tothe version of the same mark pair in the second layer.

Alternatively, a hybrid IBO-type target may be devised, comprising afirst set of marks included in the first lithographic mask and a secondset of marks included in the second lithographic mask. FIG. 22 shows anexample of such a target design. The marks in area 50 are in the firstmask, those in areas 51 are in the second mask. The comb-shaped marksCD1− and CD1+ have line features of design width w₀−Δw₀ and w₀+Δw₀respectively, Δw0 representing a pre-defined offset of the line width inthe y-direction. In the first mask, several pairs of mirrored marks ofeach type (CD1− and CD1+) are available, for the measurement of thedistance S between centroids of these two mark types, allowing todetermine CD1y in accordance with the above-described embodiments, e.g.determine α from a linear curve fit, determine δS, determineCD1y=CD1y₀+δCD1y, with δCD1y=βδw₀, with β=α/G, G=2L/p and δw₀=δS/α.

The second mask comprises cross-wise arranged versions of asymmetricmarks having a 2-dimensional-array of via-type features in the periodicportion: the vertically arranged marks CD2+ and CD2− respectively havevia type features of design dimensions in the y-direction w_(0y)+Δw₀ andw_(0y)−Δw₀. This allows to determine CD2y according to example methods.The horizontally arranged marks CD2+ and CD2− respectively have via typefeatures of design dimensions in the x-direction w_(0x)+Δw₀ andw_(0x)−Δw₀. This allows to determine CD2x according to example methods.In addition, each mask comprises marks OVL1 and OVL2 which are dedicatedto measuring the overlay error OVL, by measuring the shift of one of themarks OVL2 with respect to one of the marks OVL1, in x and y direction.

As stated, the method according to any of the above-describedembodiments, is not only applicable to lithography and etching steps.The same method steps may be based on asymmetrical mark patternsproduced by additional process steps, wherein the mark patterns producedby these steps are not actual reproductions of the asymmetric marks, butwherein the mark patterns are derived from the asymmetric marks. Theassessment of the critical dimensions of the 1-dimensional or2-dimensional features of the asymmetrical mark patterns may be based onthe actual reproductions of these mark patterns, as produced bylithography and etch for example. This has been described above. Thesemark patterns are referred to in the context of this specification and anumber of the appended claims as “initial” mark patterns. Thisassessment may however also be based on the derived asymmetric markpatterns, or it may be based on both the initial and on the derived markpatterns. For example, a further processing step may be the depositionof a spacer on an etched comb-shaped mark pattern followed by theremoval of the original comb-shaped mark pattern, leaving a serpentinemark pattern. This is what is known as the SA1 step in a self-aligneddouble patterning process. Even though the resulting mark pattern nolonger has the shape of the original mark, the pattern has a degree ofasymmetry which allows to determine the position of the centroid andthereby the value of the parameter S as a function of the w-designvalues around w₀. S_(0ref) may again be measured at the processoperating point for this particular step. This allows to determine α, δSand δw₀ as described above. The same may be done for the SA2 step, whichis a repetition of the spacer deposition and removal process performedin SA1, but based on the serpentine structure obtained as a result ofSA1. The δS values obtained from these derived mark patterns also allowto assess the critical dimensions of the comb features of the mandrelstructure obtained by lithography and etch or by another method. Whenthese critical dimensions deviate from the nominal value w₀, thisresults in the phenomenon of pitch walking in the arrays produced byself-aligned multiple patterning. In this way, the assessment of thecritical dimensions of the comb features based on the δS value obtainedfrom patterns obtained after SA1 and SA2 is in fact an assessment ofpitch walking occurring in these patterns. This particular embodiment isdescribed in more detail hereafter. This embodiment may include targetscomprising comb-shaped marks 60 as shown in FIG. 22.

The mark 60 comprises a rectangular base portion 60 a and a periodicportion 60 b, the periodic portion comprising a plurality of mutuallyparallel features 61 forming an array. When included in a lithographicmask, this mark allows to print a pattern of the same shape as the markonto a resist layer produced on a semiconductor wafer, and to etch thewafer so that a mandrel structure 60′ of the same shape is formed on thewafer, as illustrated in FIG. 23A, having a rectangular base portion 60a′ and a periodic portion 60 b′ comprising an array of line-shapedfeatures 61′. The mark 60 is configured to produce a mandrel withspecific dimensions, defined by a width w₀ of the features 61′ and pitchp of the feature array. In other words, w₀ and p are the dimensions forwhich the mark is designed. Process instabilities or non-uniformitiesmay cause a deviation from these values, which may in turn lead to pitchwalk errors when an SAxP process is applied to the mandrel structure.Example methods aim to detect such errors.

By way of example, FIGS. 23B-23E show the processing of the mark of FIG.1 through a self-aligned quadruple patterning (SAQP) process, startingfrom the mandrel of FIG. 2A, i.e. characterized by the design dimensionsw₀ and p. When a first self-aligned patterning step (commonly referredto as SA1) is performed on this mandrel structure, spacers 62 areproduced of width s on the sidewalls of the mandrel structure as shownin FIG. 23B. After that, the mandrel structure itself is removed. Asshown in FIG. 23C, this leaves a serpentine spacer structure 63 at thelocation of the features, and a line feature 64 on the opposite side ofthe original mandrel structure. A further self-aligned patterning step(SA2) repeats the procedure using the structures of FIG. 23C as amandrel, see FIG. 23D, i.e. a spacer 65 of width s is produced on thesidewalls of the first serpentine structure 63 and of the line feature64. When the serpentine mandrel 63 and the line feature 64 are removed,as shown in FIG. 23E, a double set of intertwined serpentines 66 isobtained to one side of the original mark, and a double line 67 to theother side. FIG. 23E represents the final step of the SAQP process,whereby the periodic mandrel elements of FIG. 23A are transformed to anarray of the spacer material having four times the spatial frequency ofthe original mandrel array. Critical to this transformation is awell-controlled dimensional relationship among the original mandrelpitch (p), the original mandrel element widths (w₀), and the spacerwidths (s). Self-aligned processes to produce multiples of spatialfrequency other than four may include different relationships among thevarious pattern elements to ensure a resulting regularly periodicstructure, but the methodology is similar. In all cases, the spacerformation creates connecting loops at the ends of the periodic mandrelelements that run parallel to the periodicity.

FIG. 24 first shows simulated intensity profiles representative ofoptical images corresponding to the consecutive structures illustratedin FIGS. 23A to 23E for the case of w=48 nm, p=128 nm s=16 nm. Thecurves are numbered with the numerals 2 a-2 e which correspond to therespective figures. It is seen that the images of the structuresobtained after removal of the mandrel, represented by curves 23 c-23 e,all have a first minimum 25 in the intensity profile, which correspondsto the line features 64,65 and 67 on the left-hand side of thestructures, and a second minimum 26. For simulated structures, thissecond minimum corresponds to the centroid of the serpentine structures63 and 66. The intensity at this second minimum 26 depends on thedensity of the spacer pattern making up the various serpentinestructures. The higher the spacer pattern density, the greater is theintensity contrast corresponding to the serpentine structure. For theperfectly executed SAQP process, the serpentine structures 63 and 66 aresymmetric. This means that the second intensity minimum 26 is located atthe geometrical midpoint in the x-direction, of the serpentinestructures 63 and 66. On a real structure (not simulated), a positionrepresentative of the intensity minimum is the centroid of theserpentine structure in the x-direction, detectable by an opticalmetrology tool, in the manner described above.

In accordance with example embodiments, the position along the x-axis ofthe centroid of the serpentine structures is highly sensitive to a shiftin the width w of the original features 60′ of the mandrel structure.This sensitivity is used to create a metrology target that allows tomonitor pitch walk. The method for monitoring pitch walk according toexample embodiments is based on the inclusion of one or more metrologytargets in a lithographic mask, each target comprising one or moreasymmetric comb-shaped marks of the type described above. The centroidof the serpentine structures obtained by self-aligned multiplepatterning on the basis of these marks is detected and the position ofthese centroids and/or of the centroids of the mandrel structuresproduced on the basis of the comb-shaped marks, in a directiontransverse to the periodicity of the arrays contained in the serpentinestructures is determined and evaluated with respect to a condition of noor essentially no pitch walk.

The target shown in FIG. 25 is applicable in example embodiments forcharacterizing pitch walk in an array of features produced by SAQP onthe basis of a mandrel array with features of a nominal design width w₀,and nominal design pitch p. The nominal design dimensions w₀ and p maybe used to obtain uniform periodicity in the arrays produced by SAQP onthe basis of the mandrel. The target comprises a first pair of identicaland symmetrically arranged marks 70 and 70′ and a second pair ofidentical and symmetrically arranged marks 71 and 71′, each having thecomb-shape described above in relation to FIGS. 23 and 24. Marks 70′ and71′ are the mirror images of the marks 70 and 71 about an axis that isparallel to the direction of the arrays of line features (i.e. thedirection perpendicular to the line features themselves). The first pairof marks 70/70′ are designed to produce a mandrel array with pitch p andfeatures that have a width of w₀−Δw with Δw a given value. The secondpair of marks 71/71′ are designed to produce a mandrel array with pitchp and features that have a width of w₀+Δw. For example, when w₀ is equalto 48 nm and p equal to 128 nm, as in the example described in theintroduction, Δw may be 16 nm, i.e. the first mark 70 is designed toproduce a mandrel array with 32 nm features at 128 nm pitch and thesecond mark 71 is designed to produce a mandrel array with 64 nmfeatures also at 128 nm pitch. The asymmetric versions 70 and 70′ on theone hand and 71 and 71′ on the other, are designed to be placed withrespect to each other at a distance A which is the same for the pair70/70′ as for the pair 71/71′.

FIG. 26 shows the simulated intensity profiles, equally numbered 70 and71, of the two printed and etched mandrel structures obtained from themarks 70 and 71, obtainable through the use of optical metrology tool asdescribed above, in the case of w=48 nm and Δw=16 nm. The backgroundintensity is normalized to a value of 1 at maximum intensity (i.e.outside of the mandrel structure) and patterned regions reduce the imageintensity relative to the background. It is seen that the positioncorresponding to an intensity of 0.5 shifts as a function of the Δwvalue. This means that the centroid position of the printed and etchedmandrel structures is sensitive to the design width shift Δw.

FIGS. 27 and 28 show details of the structures obtained after SA1 andSA2, as well as the corresponding intensity profiles. In both cases, itis seen that the image intensity centroid of the serpentine structureimage (in simulated structures, this corresponds to the minimum 26 inFIG. 24) shifts in the x-direction as a function of Δw. This is aconsequence of the inherent asymmetry introduced between the left-handside 75 and right-hand side 76 of the serpentines when Δw≠0, asillustrated in the details of the line-end loops shown in FIGS. 27 and28. The detection and measurement of these centroid shifts allows todetect pitch walking errors on the basis of the target of FIG. 25.

When the structures in the target of FIG. 25 are produced by lithographyand etch, followed by SA1 and SA2, the shifts of the centroids of thecomb structures of FIG. 25 and the serpentine structures of FIG. 29 canbe measured in an IBO tool, which is well-equipped for measuring thedistance between two structures in the field of view of the tool. Inparticular, the distances S(+Δw) and S(−Δw) are measured, as illustratedin FIG. 29A, which illustrates the parameters S(+Δw) and S(−Δw) afterlithography and/or etch of the mandrel structures, and in FIG. 29B afterSA1 (upper two images) and SA2 (lower two images). In FIG. 29A, S(−Δw)is the distance between the centroids of the pair of mandrel structures70/70′ (feature design width w₀−Δw), and S(+Δw) is the distance betweenthe centroids of the pair of mandrel structures 71/71′ (feature designwidth w₀₊Δw). In FIG. 29B, S(−Δw) is the distance between the centroidsof the serpentine structures generated from the symmetrically arrangedmandrels 70/70′ (feature design width w₀−Δw), S(+Δw) is the distancebetween the centroids of the serpentine structures generated from thesymmetrically arranged mandrels 71/71′ (feature design width w₀+Δw). Theprevious paragraph thus defines the parameter S in this particularembodiment.

S(−Δw) and S(+Δw) are subsequently compared to a reference valueS_(0ref) (a different reference value is to be taken into account ateach patterning step; e.g. mandrel lithography, mandrel etch, SA1 andSA2) that represents the process operation point, which is essentiallythe condition of “no pitch walk error”. The reference value may bedetermined by a calibration process performed on the lithographic maskthat is to be used in example methods, i.e. a mask that includes one ormore targets in accordance with FIG. 25 (or other suitable designs asdescribed further in this text). The mask may further comprise patternsfor producing semiconductor device features which are to be manufacturedon a chip. Some of these patterns are mandrel patterns for producingarrays of device features associated with the target. This means thatthe mandrel patterns are designed to produce mandrel structurescharacterized by a pitch that is equal or similar to the nominal pitch pand by a feature width that is equal or similar to the nominal width w₀.The associated feature arrays are produced on the basis of the mandrelstructures by the same self-aligned multiple patterning steps for whichthe target is designed (for example SA1 and SA2 in the case of SAQP).The associated device arrays may be located in the vicinity of a targetdesigned for p and w₀ (e.g. the target of FIG. 25).

When the target of FIG. 25 is used, one of the mandrel patterns forproducing associated feature arrays is itself a metrology target 80shown in FIG. 30. The target 80 may be located in the vicinity of thetarget of FIG. 25 and may be included in the target, thereby resultingin the target of FIG. 4. The target 80 comprises a pair of comb-shapedmarks 81/81′ designed for pitch p and feature width w₀, having the samedimensions of the uniform and periodic portions 60 a and 61 b as themarks in FIG. 25, and designed to be placed at the same mutual distanceA as the marks in FIG. 25.

In the calibration process, the mask patterns (device patterns andtargets) are printed, etched and processed by self-aligned multiplepatterning on one or more test wafers through a range of processingparameters. The processing parameters which are modulated through aparticular range may include the dose and focus of the lithographyprocess for printing the first mandrel structures, and the etchparameters applied in the etching process of the first mandrels, as wellas process parameters applied in the self-aligned patterning steps. Thefocus-parameter is the deviation from a pre-defined zero-defocus settingon the exposure tool, expressed for example in nm. Dose is defined asthe energy applied through the mask during exposure, expressed forexample in mJ/cm². Both focus and dose are values that can be set on thelithography tool. Etch process parameters that may be modulated are:etch time, etch rate, voltage, chamber pressure, etc. SA1 and SA2 spacerdeposition parameters may also be modulated as part of the optimizationmatrix.

The calibration procedure includes printing and etching the targetpatterns and the associated mandrel patterns multiple times on one ormore test wafers, followed by the SA1 and SA2 process steps, each timeapplying different process parameters, incrementing for example themandrel etch time in fixed steps while the SA depositions remainconstant and vice versa. To determine the optimized process operatingpoint, the associated device patterns produced by SAxP are measured byhigh resolution measurement techniques such as SEM (scanning electronmicroscopy), AFM (atomic force microscopy) or TEM (transmission electronmicroscopy), and the optical targets are measured on an IBO tool,including at least the measurement of the S-value on the target 80, i.e.the distance between the centroids of the two marks of target 80. The Svalue is measured after each step of the process: lithography, etch,SA1, SA2, etc.

The calibration procedure results in a “golden standard”, which is a setof process conditions determined on the basis of the criterion ofessentially no pitch walk errors occurring in one or more of theassociated device feature arrays obtained after completion of theself-aligned patterning steps (SA1, SA2, etc.). At these processconditions, the S values determined on the target 80 after lithography,after etch, after SA1, after SA2, etc., are taken as respectivereference S_(0ref) values.

Returning to the target of FIG. 25, the values S(+Δw) and S(−Δw)obtained after lithography, etch, SA1 and SA2 during a manufacturingprocess, are compared to their corresponding reference values S_(0ref)by calculating a value δS defined as:

$\begin{matrix}{{\delta \; S} = {\frac{{S\left( {{+ \Delta}\; w} \right)} + {S\left( {{- \Delta}\; w} \right)}}{2} - S_{0\; {ref}}}} & (34)\end{matrix}$

This is an alternative way of calculating δS appearing in equation (4),when S₀ is not explicitly measured on a mark pair as shown in FIG. 30,i.e. designed for w=w₀. Based on the linear relation between S and w, S₀may indeed be written as:

$S_{0} = \frac{{S\left( {{+ \Delta}\; w} \right)} + {S\left( {{- \Delta}\; w} \right)}}{2}$

When the structures based on the target of FIG. 25 have been producedunder conditions which do not result in a pitch walk error or very lowpitch walk error, δS equals zero or essentially zero. Therefore, themonitoring of δS allows to monitor the occurrence of pitch walk errorsduring a fabrication process. According to the above-describedembodiment, the δS values after lithography, after etch, after SA1 andafter SA2 are monitored. According to another embodiment, only the δSvalues after one or more of the self-aligned patterning steps aremonitored. According to still another embodiment, only the δS valuesafter lithography and/or etch are monitored.

According to example embodiments, a second parameter may be derived fromthe target of FIG. 25. This is the parameter α defined as the rate ofchange of the S-values as a function of the change in the feature widthw.

$\begin{matrix}{\alpha = \frac{dS}{dw}} & (35)\end{matrix}$

In accordance with example embodiments, the relationship between themeasured S parameters (after lithography, etch, SA1, SA2, etc.) and w isapproximately linear in an area around the design value w₀ thatcorresponds with realistic error margins. FIG. 31 shows SEM images of aplurality of serpentine structures obtained after SA2, for a number ofdifferent values of the feature width w: 36 nm, 42 nm, 48 nm, 54 nm, 64nm, which include a nominal value w₀ (48 nm in this case), and a numberof values below and above the nominal value w₀. The graph in FIG. 32shows the S value as a function of these w-values, and the bestapproximation of the data, indicating that the relation is largelylinear. The parameter α can be calculated from the measurement of S(+Δw)and S(−Δw) derived from the target in FIG. 25, as follows:

$\begin{matrix}{\alpha = \frac{{S\left( {{+ \Delta}\; w} \right)} - {S\left( {{- \Delta}\; w} \right)}}{2\Delta \; w}} & (36)\end{matrix}$

Alternatively, if additional mark pairs with different design widths ware available in the target, α can be determined based on a best fit ofthe S-values, as described above in relation to FIG. 5 and equation (2).

As described above, α may be compared to a reference value α₀,determined at the process operating point. In this way, α becomes asecond control parameter derivable from the target of FIG. 25. Duringmanufacturing, the value of |α−α₀| may then be compared to a tolerance.

As described also in relation to the previous embodiments, the α-valuemay be used to calculate the effective shift in the design width wcorresponding to the measured S variation:

δw ₀ =δS/α  (37)

This shift is to be interpreted as follows: when δS is higher than zero,it is as if the arrays produced by the lithography, etch or SAx stepsare derived from a mandrel with design width w₀+δw₀. Likewise, when δSis lower than zero, it is as if the arrays are derived from a mandrelwith design with w₀−δw₀. To counteract the pitch walk errors, anadjustment of the actual design width by a value of −δw₀ could beconsidered, i.e. a re-design of the mask. In some embodiments, however,process parameters are adjusted based on known relations between suchparameters and the critical dimensions obtained by lithography, etchingand self-aligned patterning. Such relations are obtained throughcalibration procedures which may not be described here in detail. Inshort, the information provided in the form of the values δS and α allowthe measurements for counteracting a detected pitch walk error thatexceeds an allowable level.

The method for monitoring pitch walk is not limited to the use of thetarget design shown in FIG. 25. The centroid shift of the asymmetricloop may end in the serpentine structures obtained from the self-alignedpatterning steps, can be measured with respect to a given offsetstructure. This offset structure may be a symmetrically mirrored version(70′,71′) of the comb mark patterns 70 and 71, as in the case of FIG.25. This technique may be used, as it provides a high sensitivity of themeasured S-value, given that S changes by twice the value of a singlecentroid shift. Alternatively, the comb mark patterns 70 and 71 could bereferenced to a symmetric offset mark pattern 72, as illustrated in thealternative design shown in FIG. 33. The symmetric mark patterns 72 aresegmented in order to allow for the creation of symmetric arraystructures by the SAxP steps, the symmetric structures having theircentroid at a fixed position at the geometrical midpoint of the arrays.The S values measured on the basis of this design allow to calculate thesame parameters δS and α as described above (with S_(0ref) obtainablefrom a calibration of the target design of FIG. 33). According to anembodiment, a target could comprise a single symmetric segmented mark72, for example placed at the center of the target and serving also as apositioning reference. The asymmetric marks designed for w₀−Δw and w₀+Δware then placed around the central mark, for example placedsymmetrically on either side of the central symmetric mark, allowing themeasurement of S(−Δw) and S(+Δw) in the manner described above.

According to example embodiments, the target applied in the method formonitoring pitch walk comprises not only the marks 70 and 71 designedfor widths w₀−Δw and w₀+Δw, but also the same comb mark designed for thenominal width w₀ and pitch p (hereafter referred to as “w₀” mark). Inthis case the target may comprise a pair of mirrored marks 81/81′designed for w₀ and p (as shown in FIG. 30) or a “w₀”—mark 81 and asymmetric offset mark 72. The target may further comprise additionalversions of the comb mark with other design values of the feature widthw (i.e. other values of +Δw and/or −Δw). An example of the latter isdescribed above and shown in FIG. 6.

In another alternative, a target suitable for the method for monitoringpitch walk errors may comprise multiple sets of comb-shaped marks, eachset defined by different nominal values of p and/or w₀ tailored toalternate SAxP process conditions.

Based on the above, example embodiments concern a method for verifyingpitch walk during the manufacturing process of semiconductor structuresby a self-aligned multiple patterning process. On the lithographic maskthat is used for producing the mandrel structures of the SAxP arrays,one or more targets are included which define reference structures thatare to be printed in the vicinity of arrays of semiconductor devicefeatures, at least some of which are associated to the targets, i.e.these device arrays are obtained by multiple patterning based on mandrelstructures designed to have features widths and pitch essentially equalor similar to w₀ and p. Each target comprises at least two marks 70 and71 of the type shown in FIG. 25, the two marks being configured forproducing mandrel structures with pitch p and shifts −Δw and +Δwrespectively, relative to a nominal feature width w₀, used for producinguniform SAxP periodicity.

During the manufacturing process, the values S(+Δw) and S(−Δw) aremeasured at one or more of the SAxP process steps, and/or afterlithography and etch (or only after lithography or only after etch) ofthe mandrel structures, in each of the targets distributed across theprinted area, and the δS-values are computed, using S_(0ref) valuespreviously determined by a calibration process. In some embodiments,also the α values are computed. The evaluation of δS derived from markscharacterized by a given set of p,w₀,s values in a particular target,with respect to a tolerance, allows to assess the degree of pitch walkoccurring in SAx-produced arrays on the wafer, which are located in thevicinity of the target. The “evaluation of δS with respect to atolerance” entails at least the evaluation of the absolute value of δSwith respect to the tolerance. As stated above, the sign of δS may thenbe used to derive a correction of the design width w₀.

When |δS| approaches or exceeds the tolerance, process parameters may beadjusted, such as the dose and focus values during lithography orparameters which influence the etch bias during etching of themandrel-structure, in order to maintain an acceptable level of pitchwalk errors across the printed area. The α value provides an additionalcriterion for monitoring stability of the production process and/or themetrology process (i.e. the measurement of S-values in an IBO tool),comparing a to a constant reference value α_(ref) (see above).

The obtained δS and α values during manufacturing may be applied in avariety of feedback control strategies and embodiments are not limitedto any particular strategy. According to one embodiment, the control isperformed on a wafer-to-wafer or possibly a chip-to-chip basis:

-   -   S(+Δw) and S(−Δw) are measured on the basis of a plurality of        targets distributed across one or more lithographic masks used        in the manufacturing process of a chip,    -   A plurality of δS values are calculated (e.g. after lithography,        after etch, after SA1, after SA2, etc.)    -   the δS values are evaluated with respect to a tolerance,    -   when δS values appear which approach or exceed the tolerance,        one or more process parameters are adjusted in the production        process of the next wafer or chip. This may be an adjustment of        dose during the lithography process, and/or an adjustment of        etch parameters during the etch process for producing the        mandrel structures, or possibly an adjustment of the deposition        parameters applied during the self-aligned patterning steps        (SA1, SA2, etc.). As stated above, the precise adjustments used        to correct the pitch walk error can be determined by the person        skilled in the art of lithography/etch and SAxP processes, and        may not be described here in detail.

If the same targets are used in different layers of a chip, a feedbackcontrol may be realized during the processing of a single chip: pitchwalk occurring in one layer may then be compensated for in one of thefollowing layers of the chip.

It is also possible to adjust processing parameters during the steps ofthe self-aligned patterning process. For example when δS, determinedafter SA1 is too high, the SA2 process may be adjusted. In this way, thecontrol strategy applied is a feed-forward control.

Example methods may also be applied in patterning steps performed in theproduction of a lithographic mask as such. For example, an EUV mask isproduced on an EUV blank by patterning an absorber layer on the blank.By including one or more metrology targets in accordance with exampleembodiments, the critical dimensions of the features as reproduced onthe absorber layer may be monitored.

Example methods are not only applicable to processes involving alithographic mask, but it is applicable to any process step wherein apattern of features is reproduced on a substrate, according to apredefined pattern. This includes steps applying the techniques known as“maskless lithography” or “direct writing”. What counts is that at leastone target comprising one or more asymmetric mark as described above isincluded in the predefined pattern, so that the marks are reproduced ona substrate as mark patterns, from which δS can be determined.

The method is also not limited to process steps involving a devicelayout in a chip manufacturing process, but it is also applicable toprocesses which do not directly result in a micro-electronic device, forexample to the printing of test patterns used for patterning tool orprocess qualification.

According to example embodiments, when δw₀ and/or δCD (in x and/or in ydirection) approaches or exceeds a given tolerance, process parametersmay be adjusted, such as the dose and focus values during lithography orone or more etching parameters, in order to counteract the deviationfrom the design width w₀. Depending on the precise definition of theS-parameter, the sign of δw₀ or δCD indicates whether certain parametersshould be increased or decreased for counteracting the deviation fromthe design width. The α and β values provide an additional criterion formonitoring stability of the production process and/or the metrologyprocess.

The obtained δw₀, δCD, α, and β values may be applied in a variety offeedback control strategies and embodiments are not limited to anyparticular strategy. According to one embodiment, the control isperformed on a chip-to-chip basis, described hereafter for theevaluation based on δw₀, determined from targets comprising opticalmarks (IBO e.g.) but valid also for evaluations based on DBO and/or δCD,α or β:

-   -   δw₀ is measured on the basis of a plurality of targets        distributed across one or more lithographic masks used in the        manufacturing process of a chip,    -   the δw₀ values are evaluated with respect to a tolerance,    -   when δw₀ values appear which approach or exceed the tolerance,        one or more process parameters are adjusted in the production        process of the next chip. This may be an adjustment of focus and        dose during the lithography process, and/or an adjustment of        etch parameters during the etch process. The precise adjustments        used to correct the error can be determined by the person        skilled in the art of lithography/etch processes, and may not be        described here in detail.

If the same targets are used in different layers of a chip, a feedbackcontrol may be realized during the processing of a single chip:deviations from w₀ in one layer may then be corrected in one of thefollowing layers of the chip.

Embodiments are equally related to a computer-implemented monitoringunit configured to be integrated in an apparatus for semiconductorprocessing including lithography and etching steps, the apparatusfurther comprising an optical metrology tool, for example an image basedor diffraction based overlay tool. The monitoring unit is basically acomputer configured to execute the method steps as set out in any of theappended claims.

According to example embodiments, the monitoring unit comprises a memoryprovided with a computer program for executing the method steps, whenthe program is run on the monitoring unit. Embodiments are equallyrelated to a software product configured to execute the above methodsteps, when the program is run on the monitoring unit.

The monitoring unit may be configured to calculate—on the basis of theresults of the assessment—updated processing parameters of one or moreprocesses performed by the semiconductor processing apparatus, andfeedback the updated parameters to the apparatus.

While embodiments have been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative and not restrictive. Other variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art in practicing the claims, from a study of thedrawings, the disclosure, and the appended claims. In the claims, theword “comprising” does not exclude other elements or steps, and theindefinite article “a” or “an” does not exclude a plurality. The merefact that certain measures are recited in mutually different dependentclaims does not indicate that a combination of these measures cannot beused. Any reference signs in the claims should not be construed aslimiting the scope.

What is claimed is:
 1. A method for monitoring critical dimensions of1-dimensional features or 2-dimensional features arranged in a patternand produced on a substrate by a process step that is part of or relatedto a manufacturing process for producing a semiconductor device, whereinthe process step is performed in accordance with a predefined patterndesign, wherein one or more metrology targets are added to thepredefined pattern design, wherein each metrology target comprises oneor more marks of an asymmetric design, wherein each mark of theasymmetric design comprises a uniform portion of width K and a periodicportion of width L arranged adjacently in a given direction, wherein theperiodic portion comprises 1-dimensional features or 2-dimensionalfeatures arranged in a regular array, wherein the periodic portioncomprises an array of 1-dimensional features or 2-dimensional featuresat a design pitch, wherein the design dimension of the 1-dimensionalfeatures or one of the design dimensions of the 2-dimensional featuresis equal to or situated in a range around a nominal value w₀, andwherein the method comprises each of the following steps performed inrelation to each of the one or more metrology targets: performing theprocess step, thereby obtaining one or more asymmetric mark patternscorresponding respectively to the one or more marks of the asymmetricdesign; defining a parameter S representative of a position of acentroid of the one or more asymmetric mark patterns, wherein theparameter S changes essentially linearly as a function of at least oneof the design dimensions of the of the 1-dimensional or 2-dimensionalfeatures; determining, from the one or more asymmetric mark patterns, ashift δS of the parameter S for an asymmetric mark having features ofdesign dimension w₀ with respect to a reference value for the parameterS, wherein the reference value is valid at a previously defined processoperating point for the process step; and assessing, based on the shiftδS, a critical dimension of 1-dimensional or 2-dimensional features ofthe one or more asymmetric mark patterns produced by the process step.2. The method according to claim 1, wherein the method furthercomprises, for each metrology target: determining a proportionalityfactor α; calculating δw₀=δS/α; and evaluating δw₀ with respect to atolerance.
 3. The method according to claim 2, further comprisingevaluating |α−α₀| with respect to a tolerance, wherein α₀ is a value ofthe proportionality factor α at the process operating point.
 4. Themethod according to claim 2, wherein one or more of the metrologytargets comprise asymmetric marks configured so that the position of thecentroid of the one or more asymmetric mark patterns is detectable by anoptical tool, such that the parameter S can be measured using theoptical tool, wherein the one or more metrology targets comprise aplurality of the asymmetric marks, having the one or more designdimensions in a range around the nominal value, wherein, for each of theone or more metrology targets, the proportionality factor α isdetermined by fitting a linear function to measured values of theparameter S for the plurality of the asymmetric marks, wherein, for eachof the one or more metrology targets, δS is determined as S₀−S_(0ref),wherein S₀ is the parameter value for a mark pattern of a mark havingfeatures of design dimension in a range around the nominal value, andwherein S_(0ref) is the reference value for the parameter S at theprocess operating point.
 5. The method according to claim 2, wherein theone or more asymmetric marks are arranged in each of the one or moremetrology targets as mirrored pairs of identical marks, wherein themirror pairs of identical marks are mirrored about a line perpendicularto a direction in which the uniform portion and the periodic portion arearranged, and wherein the parameter S is a function of a distance Dbetween the centroids of the two mirrored mark patterns representativeof the two mirrored marks.
 6. The method according to claim 2, whereinthe one or more asymmetric marks are arranged in each of the one or moremetrology targets with respect to one or more symmetric marks, andwherein the parameter S is a function of a distance D between thecentroid of the asymmetric mark patterns representative of or derivedfrom the one or more asymmetric marks and the centroid of a symmetricmark pattern or patterns representative of the one or more symmetricmarks.
 7. The method according to claim 6, wherein the positionparameter S is equal to D−D_(s), wherein D_(s) is a design value of adistance between geometric midpoints of two mirrored marks, and whereinD_(s) is a constant for each of the design dimensions and for the givendirection.
 8. The method according to claim 2, wherein one or more ofthe metrology targets comprises at least a first set of diffractiongratings and a second set of diffraction gratings, wherein each set ofdiffraction gratings is comprises two grating marks arranged adjacentlyin the same direction and in a repeated manner, wherein at least one ofthe grating marks in each set is one of the asymmetric marks, whereinthe asymmetric mark of the first grating set comprises features ofdesign dimension w₀−Δw₀, wherein Δw₀ is a predefined offset value,wherein the asymmetric mark of the second grating set comprises featuresof design dimension w₀+Δw₀, wherein δS and α are calculated from a firstequation and a second equation, wherein the first equation isα=(ΔI₁−ΔI₂)/2κΔw₀, wherein the second equation is δS=(ΔI₁+ΔI₂)/2κ,wherein ΔI₁ and ΔI₂ are differences between a plus and a minus firstorder diffracted intensity measured on a first set of mark features anda second set of mark features obtained by the lithography step or theetch step, based on the first grating set and second grating set, andwherein κ is a diffraction factor.
 9. The method according to claim 8,wherein κ is calculated from a measurement of the plus and the minusdiffraction intensities of an additional set of two gratings locatedwithin or in the vicinity of the pattern design, wherein each gratingset comprises two marks arranged adjacently and in a repeated manner,wherein at least one of the two marks arranged adjacently comprises anasymmetric mark, wherein a distance between the two adjacent marks isdifferent in the first grating compared to the second grating, andwherein the difference between the distances in the first grating andthe second grating is pre-defined.
 10. The method according to claim 2,further comprising, for each metrology target and for at least one ofthe dimensions: calculating, from the shift δS, a shift δCD of thecritical dimension CD of features of the nominal design dimension w₀,with respect to the value of the critical dimension CD_(p) at theprocess operating point, wherein the shift δCD of the critical dimensionCD is calculated as βδw₀, and wherein β is a proportionality factor thatexpresses a linear relation between the critical dimension CD and thedesign dimension of the features; and evaluating the shift δCD of thecritical dimension CD with respect to a tolerance.
 11. The methodaccording to claim 10, wherein the proportionality factor β iscalculated as (α/G), wherein G is proportional to L/p, and wherein L isa width in the direction of asymmetry of the periodic portion of themark pattern or patterns and p is a nominal pitch of line features ofthe periodic portion.
 12. The method according to claim 10, furthercomprising the step of evaluating |β−1| with respect to a tolerance. 13.The method according to claim 1, wherein the one or more asymmetricmarks in at least one of the metrology targets are comb-shaped marks,and wherein the periodic portion comprises an array of parallel featuresextending in the given direction in which the uniform portion and theperiodic portion are arranged.
 14. The method according to claim 1,wherein the one or more asymmetric marks in at least one of themetrology targets are rail-shaped marks, and wherein the periodicportion comprises an array of parallel features extending in a directionperpendicular to the given direction in which the uniform portion andthe periodic portion are arranged.
 15. The method according to claim 1,wherein at least one of the metrology targets comprises a first group ofasymmetric marks, of which the uniform portion and the periodic portionare arranged in a first direction, and a second group of the sameasymmetric marks, of which the uniform portion and the periodic portionare arranged in a second direction perpendicular to the first direction,and wherein the shift δS is determined for each of the first directionand the second direction.
 16. The method according to claim 15, whereinone or more of the metrology targets comprises asymmetric marksconfigured so that the position of the centroid of the asymmetric markpatterns is detectable by an optical tool, such that the parameter S canbe measured using the optical tool, and wherein the asymmetric marks arearranged in cross-shapes, each cross comprising four identicalasymmetric marks.
 17. The method according to claim 15, wherein the atleast one metrology target comprises at least one first area and atleast one second area, and wherein the asymmetric marks in the secondarea are mirrored with respect to the asymmetric marks in the firstarea.
 18. The method according to claim 1, wherein the process step is alithography step using a lithographic mask or an etch step following thelithography step, and wherein the one or more metrology targets areincluded in the lithographic mask.
 19. A method for monitoring criticaldimensions of 1-dimensional features or 2-dimensional features arrangedin a pattern and produced on a substrate by an initial process step thatis part of or related to a manufacturing process for producing asemiconductor device, wherein the initial process step is performed inaccordance with a predefined pattern design, wherein one or moremetrology targets are added to the pattern design, wherein eachmetrology target comprises one or more marks of an asymmetric design,wherein each mark of the asymmetric design comprises a uniform portionof width K and a periodic portion of width L arranged adjacently in agiven direction, wherein the periodic portion comprises 1-dimensional or2-dimensional features arranged in a regular array, wherein the periodicportion comprises an array of 1-dimensional features or 2-dimensionalfeatures at a design pitch, wherein the design dimension of the1-dimensional features or one of the design dimensions of the2-dimensional features is equal to or situated in a range around anominal value w₀, wherein the method comprises additional process stepsafter the initial process step, thereby obtaining one or more asymmetricmark patterns not directly corresponding to the one or more asymmetricmarks but derived from the one or more asymmetric marks, and wherein themethod comprises each of the following steps performed in relation toeach of the one or more metrology targets: performing the initialprocess step, thereby obtaining one or more asymmetric mark patternscorresponding respectively to the one or more marks of the asymmetricdesign; defining a parameter S representative of a position of acentroid of the one or more asymmetric mark patterns, wherein theparameter S changes essentially linearly as a function of at least oneof the design dimensions of the of the 1-dimensional or 2-dimensionalfeatures; determining, from the one or more asymmetric mark patterns, ashift δS of the parameter S for an asymmetric mark having features ofdesign dimension w₀ with respect to a reference value for the parameterS, wherein the reference value is valid at a previously defined processoperating point for the initial process step; and assessing, based onthe shift δS, a critical dimension of 1-dimensional or 2-dimensionalfeatures of the one or more asymmetric mark patterns produced by theinitial process step.
 20. A computer-implemented monitoring unitconfigured to be integrated in an apparatus for semiconductor processingincluding lithography and etching steps, wherein the apparatus comprisesa metrology tool, wherein the monitoring unit is configured to executethe steps of a method for monitoring critical dimensions of1-dimensional features or 2-dimensional features arranged in a patternand produced on a substrate by a process step that is part of or relatedto a manufacturing process for producing a semiconductor device, whereinthe process step is performed in accordance with a predefined patterndesign, wherein one or more metrology targets are added to thepredefined pattern design, wherein each metrology target comprises oneor more marks of an asymmetric design, wherein each mark of theasymmetric design comprises a uniform portion of width K and a periodicportion of width L arranged adjacently in a given direction, wherein theperiodic portion comprises 1-dimensional features or 2-dimensionalfeatures arranged in a regular array, wherein the periodic portioncomprises an array of 1-dimensional features or 2-dimensional featuresat a design pitch, wherein the design dimension of the 1-dimensionalfeatures or one of the design dimensions of the 2-dimensional featuresis equal to or situated in a range around a nominal value w₀, andwherein the method comprises each of the following steps performed inrelation to each of the one or more metrology targets: performing theprocess step, thereby obtaining one or more asymmetric mark patternscorresponding respectively to the one or more marks of the asymmetricdesign; defining a parameter S representative of a position of acentroid of the one or more asymmetric mark patterns, wherein theparameter S changes essentially linearly as a function of at least oneof the design dimensions of the of the 1-dimensional or 2-dimensionalfeatures; determining, from the one or more asymmetric mark patterns, ashift δS of the parameter S for an asymmetric mark having features ofdesign dimension w₀ with respect to a reference value for the parameterS, wherein the reference value is valid at a previously defined processoperating point for the process step; and assessing, on the basis of theshift δS, a critical dimension of 1-dimensional or 2-dimensionalfeatures of the one or more asymmetric mark patterns produced by theprocess step.